BASIC DATA TYPES
Eight different data types exist in PHP, five of which are scalar and each of the
remaining three has its own uniqueness. The previously discussed variables
can contain values of any of these data types without explicitly declaring their
type. The variable “behaves” according to the data type it contains.
2.5.1 IntegersIntegersare whole numbers and are equivalent in range as your C compiler’s
longvalue. On many common machines, such as Intel Pentiums, that means a
32-bit signed integer with a range between –2,147,483,648 to +2,147,483,647.
Integers can be written in decimal, hexadecimal (prefixed with 0x), and
octal notation (prefixed with 0), and can include +/-signs.
Some examples of integers include
240000
0xABCD
007
-100
Note:As integers are signed, the right shift operator in PHP always does a
signed shift.
2.5.2 Floating-Point Numbers
Floating-point numbers(also known as real numbers) represent real
numbers and are equivalent to your platform C compiler’s doubledata type.
On common platforms, the data type size is 8 bytes and it has a range of
approximately 2.2E–308 to 1.8E+308. Floating-point numbers include a deci-mal point and can include a +/-sign and an exponent value.
Examples of floating-point numbers include
3.14
+0.9e-2
-170000.5
54.6E42
2.5.3 StringsStringsin PHP are a sequence of characters that are always internally null-terminated. However, unlike some other languages, such as C, PHP does not
rely on the terminating null to calculate a string’s length, but remembers its
length internally. This allows for easy handling of binary data in PHP—for
example, creating an image on-the-fly and outputting it to the browser. The
maximum length of strings varies according to the platform and C compiler,
but you can expect it to support at least 2GB. Don’t write programs that test
this limit because you’re likely to first reach your memory limit.
When writing string values in your source code, you can use double
quotes ("), single quotes (') or here-docs to delimit them. Each method is
explained in this section. 2.5.3.1 Double Quotes Examples for double quotes:
"PHP: Hypertext Pre-processor"
"GET / HTTP/1.0\n"
"1234567890"
Strings can contain pretty much all characters. Some characters c
written as is, however, and require special notation:
An additional feature of double-quoted strings is that certain nota
variables and expressions can be embedded directly within them. W
going into specifics, here are some examples of legal strings that embe
ables. The references to variables are automatically replaced with th
ables’ values, and if the values aren’t strings, they are converted t
corresponding string representations (for example, the integer 123wo
first converted to the string "123").
"The result is $result\n"
"The array offset $i contains $arr[$i]"
In cases, where you’d like to concatenate strings with values (such
ables and expressions) and this syntax isn’t sufficient, you can use the .(do
ator to concatenate two or more strings. This operator is covered in a later
2.5.3.1 Double Quotes Examples for double quotes:
"PHP: Hypertext Pre-processor"
"GET / HTTP/1.0\n"
"1234567890"
Strings can contain pretty much all characters. Some characters can’t be
written as is, however, and require special notation:An additional feature of double-quoted strings is that certain notations of
variables and expressions can be embedded directly within them. Without
going into specifics, here are some examples of legal strings that embed vari-
ables. The references to variables are automatically replaced with the vari-
ables’ values, and if the values aren’t strings, they are converted to their
corresponding string representations (for example, the integer 123would be
first converted to the string "123").
"The result is $result\n"
"The array offset $i contains $arr[$i]"
In cases, where you’d like to concatenate strings with values (such as vari-
ables and expressions) and this syntax isn’t sufficient, you can use the .(dot) oper-
ator to concatenate two or more strings. This operator is covered in a later section.
\n Newline.
\t Tab.
\" Double quote.
\\ Backslash.
\0 ASCII 0 (null).
\r Line feed.
\$ Escape $sign so that it is not treated as a variable but as the
character $.
\{Octal #} The character represented by the specified octal #—for exam-
ple, \70represents the letter 8.
\x{Hexadecimal #} The character represented by the specified hexadecimal #—for
example, \0x32represents the letter 2.
2.5.3.2 Single Quotes In addition to double quotes, single quotes m
delimit strings. However, in contrast to double quotes, single quotes
support all the double quotes’ escaping and variable substitution.
The following table includes the only two escapings supported by
quotes:
Newline.
Tab.
Double quote.
Backslash.
ASCII 0 (null).
Line feed.
Escape $sign so that it is not treated as a variable
character $.
tal #} The character represented by the specified octal #—
ple, \70represents the letter 8.
exadecimal #} The character represented by the specified hexadec
example, \0x32represents the letter 2.
Single quote.
Backslash, used when wanting to represent a back
lowed by a single quote—for example, \\'.Examples:
'Hello, World'
'Today\'s the day'
2.5.3.3 Here-Docs Here-docsenable you to embed large pieces of text in
your scripts, which may include lots of double quotes and single quotes, with-out having to constantly escape them.
The following is an example of a here-doc:
<<PHP stands for "PHP: Hypertext Preprocessor".
The acronym "PHP" is therefore, usually referred to as a recursive acronym
➥because the long form contains the acronym itself.
As this text is being written in a here-doc there is no need to escape the
➥double quotes.
THE_END
The strings starts with <<<, followed by a string that you know doesn’t
appear in your text. It is terminated by writing that string at the beginning of
a line, followed by an optional semicolon (;), and then a required newline (\n).
Escaping and variable substitution in here-docs is identical to double-quoted
strings except that you are not required to escape double quotes.
2.5.3.4 Accessing String Offsets Individual characters in a string can be
accessed using the $str{offset}notation. You can use it to both read and write
string offsets. When reading characters, this notation should be used only to
access valid indices. When modifying characters, you may access offsets that
don’t yet exist. PHP automatically sets that offset to the said character, and if
this results in a gap between the ending of the original string and the offset of
the new character, the gap filled with space characters (' ').
This example creates and prints the string "Andi"(in an awkward way):
$str = "A";
$str{2} = "d";
$str{1} = "n";
$str = $str . "i";
print $str;
Tip: For many cases, PHP has string manipulation functions which use effi-cient algorithms. You should first look at them before you access strings
directly using string offsets. They are usually prefixed with str_. For more
complex needs, the regular expressions functions—most notably the pcre_
family of functions—will come in handyThis support still exists in PHP 5, and you are likely to bump into it often.
However, you should really use the {}notation because it differentiates string
offsets from array offsets and thus, makes your code more readable.
2.5.4 Booleans
Booleans were introduced for the first time in PHP 4 and didn’t exist in prior
versions. A Boolean value can be either trueor false.
As previously mentioned, PHP automatically converts types when
needed. Boolean is probably the type that other types are most often converted
to behind the scenes. This is because, in any conditional code such as if state-ments, loops, and so on, types are converted to this scalar type to check if the
condition is satisfied. Also, comparison operators result in a Boolean value.
Consider the following code fragment:
$numerator = 1;
$denominator = 5;
if ($denominator == 0) {
print "The denominator needs to be a non-zero number\n";
}
The result of the equal-than operator is a Boolean; in this case, it would
be falseand, therefore, the if() statement would not be entered.
Now, consider the next code fragment:
$numerator = 1;
$denominator = 5;
if ($denominator) {
/* Perform calculation */
} else {
print "The denominator needs to be a non-zero number\n";
}
You can see that no comparison operator was used in this example; how-ever, PHP automatically internally converted $denominatoror, to be more accu-rate, the value 5to its Boolean equivalent, true, to perform the if()statement
and, therefore, enter the calculation.
Although not all types have been covered yet, the following table shows
truth values for their values. You can revisit this table to check for the types of
Boolean value equivalents, as you learn about the remaining types.2.5.5 Null
Nullis a data type with only one possible value: the NULLvalue. It marks vari-
ables as being empty, and it’s especially useful to differentiate between the
empty string and null values of databases.
The isset($variable)operator of PHP returns falsefor NULL, and truefor
any other data type, as long as the variable you’re testing exists.
The following is an example of using NULL:
$value = NULL;
2.5.6 Resources
Resources, a special data type, represent a PHP extension resource such as a
database query, an open file, a database connection, and lots of other external
types.
You will never directly touch variables of this type, but will pass them
around to the relevant functions that know how to interact with the specified
resource.
2.5.7 Arrays
An arrayin PHP is a collection of key/value pairs. This means that it maps
keys (or indexes) to values. Array indexescan be either integers or strings
whereas values can be of any type (including other arrays).
Tip:Arrays in PHP are implemented using hash tables, which means that
accessing a value has an average complexity of O(1).
2.5.7.1array()construct Arrays can be declared using the array()lan-
guage construct, which generally takes the following form (elements inside
square brackets, [], are optional):
array([key =>] value, [key =>] value, ...)
Data Type False Values True Values
Integer 0 All non-zero values
Floating point 0.0 All non-zero values
Strings Empty strings ()""
The zero string ()"0"
All other strings
Null Always Never
Array If it does not contain
any elements
If it contains at least
one element
Object Never Always
Resource Never AlwaysThe key is optional, and when it’s not specified, the key is automatically
assigned one more than the largest previous integer key (starting with 0). You
can intermix the use with and without the key even within the same declara-tion.
The value itself can be of any PHP type, including an array. Arrays con-taining arrays give a similar result as multi-dimensional arrays in other lan-guages.
Here are a few examples:
☞array(1, 2, 3)is the same as the more explicit array(0 => 1, 1 => 2, 2
➥=> 3).
☞array("name" => "John", "age" => 28)
☞array(1 => "ONE", "TWO", "THREE")is equivalent to array(1 => "ONE", 2 =>
➥"TWO", 3 => "THREE").
☞array() an empty array.
Here’s an example of a nested array()statement:
array(array("name" => "John", "age" => 28), array("name" =>
➥"Barbara", "age" => 67))
The previous example demonstrates an array with two elements: Each
one is a collection (array) of a person’s information.
2.5.7.2 Accessing Array Elements Array elements can be accessed by using
the $arr[key]notation, where keyis either an integer or string expression.
When using a constant string for key,make sure you don’t forget the single or
double quotes, such as $arr["key"]. This notation can be used for both reading
array elements and modifying or creating new elements.
2.5.7.3 Modifying/Creating Array Elements
$arr1 = array(1, 2, 3);
$arr2[0] = 1;
$arr2[1] = 2;
$arr2[2] = 3;
print_r($arr1);
print_r($arr2);
The print_r()function has not been covered yet in this book, but when it
is passed an array, it prints out the array’s contents in a readable way. You can
use this function when debugging your scripts.
The previous example prints
Array
(
[0] => 1[1] => 2
[2] => 3
)
Array
(
[0] => 1
[1] => 2
[2] => 3
)
So, you can see that you can use both the array()construct and the
$arr[key] notation to create arrays. Usually, array()is used to declare arrays
whose elements are known at compile-time, and the $arr[key]notation is used
when the elements are only computed at runtime.
PHP also supports a special notation, $arr[], where the key is not speci-
fied. When creating new array offsets using this notation (fo example, using it
as the l-value), the key is automatically assigned as one more than the largest
previous integer key.
Therefore, the previous example can be rewritten as follows:
$arr1 = array(1, 2, 3);
$arr2[] = 1;
$arr2[] = 2;
$arr2[] = 3;
The result is the same as in the previous example.
The same holds true for arrays with string keys:
$arr1 = array("name" => "John", "age" => 28);
$arr2["name"] = "John";
$arr2["age"] = 28;
if ($arr1 == $arr2) {
print '$arr1 and $arr2 are the same' . "\n";
}
The message confirming the equality of both arrays is printed.
2.5.7.4 Reading array values You can use the $arr[key]notation to read
array values. The next few examples build on top of the previous example:
print $arr2["name"];
if ($arr2["age"] < 35) {
print " is quite young\n";
}This example prints
John is quite young
Note:As previously mentioned, using the $arr[]syntax is not supported
when reading array indexes, but only when writing them.
2.5.7.5 Accessing Nested Arrays (or Multi-Dimensional Arrays) When
accessing nested arrays, you can just add as many square brackets as required
to reach the relevant value. The following is an example of how you can
declare nested arrays:
$arr = array(1 => array("name" => "John", "age" => 28), array("name"
➥=> "Barbara", "age" => 67))
You could achieve the same result with the following statements:
$arr[1]["name"] = "John";
$arr[1]["age"] = 28;
$arr[2]["name"] = "Barbara";
$arr[2]["age"] = 67;
Reading a nested array value is trivial using the same notation. For
example, if you want to print John’s age, the following statement does the
trick:
print $arr[1]["age"];
2.5.7.6 Traversing Arrays Using foreach There are a few different ways of
iterating over an array. The most elegant way is the foreach()loop construct.
The general syntax of this loop is
foreach($array as [$key =>] [&] $value)
...
$keyis optional, and when specified, it contains the currently iterated
value’s key, which can be either an integer or a string value, depending on the
key’s type.
Specifying &for the value is also optional, and it has to be done if you are
planning to modify $valueand want it to propagate to $array.In most cases,
you won’t want to modify the $valuewhen iterating over an array and will,
therefore, not need to specify it.Here’s a short example of the foreach()loop:
$players = array("John", "Barbara", "Bill", "Nancy");
print "The players are:\n";
foreach ($players as $key => $value) {
print "#$key = $value\n";
}
The output of this example is
The players are:
#0 = John
#1 = Barbara
#2 = Bill
#3 = Nancy
Here’s a more complicated example that iterates over an array of people
and marks which person is considered old and which one is considered young:
$people = array(1 => array("name" => "John", "age" => 28),
➥array("name" => "Barbara", "age" => 67));
foreach ($people as &$person) {
if ($person["age"] >= 35) {
$person["age group"] = "Old";
} else {
$person["age group"] = "Young";
}
}
print_r($people);
Again, this code makes use of the print_r()function.
The output of the previous code is the following:
Array
(
[1] => Array
(
[name] => John
[age] => 28
[age group] => Young
)
[2] => Array
(
[name] => Barbara
[age] => 67
[age group] => Old)
)
You can see that both the John and Barbara arrays inside the $people
array were added an additional value with their respective age group.
2.5.7.7 Traversing Arrays Using list()and each() Although foreach()
is the nicer way of iterating over an array, an additional way of traversing an
array is by using a combination of the list()construct and the each()func-
tion:
$players = array("John", "Barbara", "Bill", "Nancy");
reset($players);
while (list($key, $val) = each($players)) {
print "#$key = $val\n";
}
The output of this example is
#0 = John
#1 = Barbara
#2 = Bill
#3 = Nancy
2.5.7.8reset() Iteration in PHP is done by using an internal array pointer
that keeps record of the current position of the traversal. Unlike with
foreach(), when you want to use each()to iterate over an array, you must
reset()the array before you start to iterate over it. In general, it is best for
you to always use foreach()and not deal with this subtle nuisance of each()
traversal.
2.5.7.9each() The each()function returns the current key/value pair and
advances the internal pointer to the next element. When it reaches the end
of of the array, it returns a booloean value of false. The key/value pair is
returned as an array with four elements: the elements 0and "key", which
have the value of the key, and elements 1and "value", which have the value
of the value. The reason for duplication is that, if you’re accessing these ele-
ments individually, you’ll probably want to use the names such as
$elem["key"]and $elem["value"]:
$ages = array("John" => 28, "Barbara" => 67);
reset($ages);
$person = each($ages);print $person["key"];
print " is of age ";
print $person["value"];
This prints
John is of age 28
When we explain how the list()construct works, you will understand
why offsets 0 and 1 also exist.
2.5.7.10list() The list()construct is a way of assigning multiple array
offsets to multiple variables in one statement:
list($var1, $var2, ...) = $array;
The first variable in the list is assigned the array value at offset 0, the
second is assigned offset 1, and so on. Therefore, the list()construct trans-lates into the following series of PHP statements:
$var1 = $array[0];
$var2 = $array[1];
...
As previously mentioned, the indexes 0 and 1 returned by each()are
used by the list()construct. You can probably already guess how the combi-nation of list()and each()work.
Consider the highlighted line from the previous $playerstraversal example:
$players = array("John", "Barbara", "Bill", "Nancy");
reset($players);
while (list($key, $val) = each($players)) {
print "#$key = $val\n";
}
What happens in the boldfaced line is that during every loop iteration,
each()returns the current position’s key/value pair array, which, when exam-ined with print_r(), is the following array:
Array
(
[1] => John
[value] => John[0] => 0
[key] => 0
)
Then, the list()construct assigns the array’s offset 0 to $keyand offset 1
to $val.
2.5.7.11 Additional Methods for Traversing Arrays You can use other func-tions to iterate over arrays including current()and next(). You shouldn’t use
them because they are confusing and are legacy functions. In addition, some
standard functions allow all sorts of elegant ways of dealing with arrays such
as array_walk(), which is covered in a later chapter.
2.5.8 Constants
In PHP, you can define names, called constants, for simple values. As the
name implies, you cannot change these constants once they represent a cer-tain value. The names for constants have the same rules as PHP variables
except that they don’t have the leading dollar sign. It is common practice in
many programming languages—including PHP—to use uppercase letters for
constant names, although you don’t have to. If you wish, which we do not rec-ommend, you may define your constants as case-insensitive, thus not requir-ing code to use the correct casing when referring to your constants.
Tip:Only use case-sensitive constants both to be consistent with accepted cod-ing standards and because it is unclear if case-insensitive constants will con-tinued to be supported in future versions of PHP.
Unlike variables, constants, once defined, are globally accessible. You
don’t have to (and can’t) redeclare them in each new function and PHP file.
To define a constant, use the following function:
define("CONSTANT_NAME", value [, case_sensitivity])
Where:
☞"CONSTANT_NAME"is a string.
☞valueis any valid PHP expression excluding arrays and objects.
☞case_sensitivityis a Boolean (true/false) and is optional. The default is
true.
An example for a built-in constant is the Boolean value true, which is
registered as case-insensitive.
Here’s a simple example for defining and using a constant:
Eight different data types exist in PHP, five of which are scalar and each of the
remaining three has its own uniqueness. The previously discussed variables
can contain values of any of these data types without explicitly declaring their
type. The variable “behaves” according to the data type it contains.
2.5.1 IntegersIntegersare whole numbers and are equivalent in range as your C compiler’s
longvalue. On many common machines, such as Intel Pentiums, that means a
32-bit signed integer with a range between –2,147,483,648 to +2,147,483,647.
Integers can be written in decimal, hexadecimal (prefixed with 0x), and
octal notation (prefixed with 0), and can include +/-signs.
Some examples of integers include
240000
0xABCD
007
-100
Note:As integers are signed, the right shift operator in PHP always does a
signed shift.
2.5.2 Floating-Point Numbers
Floating-point numbers(also known as real numbers) represent real
numbers and are equivalent to your platform C compiler’s doubledata type.
On common platforms, the data type size is 8 bytes and it has a range of
approximately 2.2E–308 to 1.8E+308. Floating-point numbers include a deci-mal point and can include a +/-sign and an exponent value.
Examples of floating-point numbers include
3.14
+0.9e-2
-170000.5
54.6E42
2.5.3 StringsStringsin PHP are a sequence of characters that are always internally null-terminated. However, unlike some other languages, such as C, PHP does not
rely on the terminating null to calculate a string’s length, but remembers its
length internally. This allows for easy handling of binary data in PHP—for
example, creating an image on-the-fly and outputting it to the browser. The
maximum length of strings varies according to the platform and C compiler,
but you can expect it to support at least 2GB. Don’t write programs that test
this limit because you’re likely to first reach your memory limit.
When writing string values in your source code, you can use double
quotes ("), single quotes (') or here-docs to delimit them. Each method is
explained in this section. 2.5.3.1 Double Quotes Examples for double quotes:
"PHP: Hypertext Pre-processor"
"GET / HTTP/1.0\n"
"1234567890"
Strings can contain pretty much all characters. Some characters c
written as is, however, and require special notation:
An additional feature of double-quoted strings is that certain nota
variables and expressions can be embedded directly within them. W
going into specifics, here are some examples of legal strings that embe
ables. The references to variables are automatically replaced with th
ables’ values, and if the values aren’t strings, they are converted t
corresponding string representations (for example, the integer 123wo
first converted to the string "123").
"The result is $result\n"
"The array offset $i contains $arr[$i]"
In cases, where you’d like to concatenate strings with values (such
ables and expressions) and this syntax isn’t sufficient, you can use the .(do
ator to concatenate two or more strings. This operator is covered in a later
2.5.3.1 Double Quotes Examples for double quotes:
"PHP: Hypertext Pre-processor"
"GET / HTTP/1.0\n"
"1234567890"
Strings can contain pretty much all characters. Some characters can’t be
written as is, however, and require special notation:An additional feature of double-quoted strings is that certain notations of
variables and expressions can be embedded directly within them. Without
going into specifics, here are some examples of legal strings that embed vari-
ables. The references to variables are automatically replaced with the vari-
ables’ values, and if the values aren’t strings, they are converted to their
corresponding string representations (for example, the integer 123would be
first converted to the string "123").
"The result is $result\n"
"The array offset $i contains $arr[$i]"
In cases, where you’d like to concatenate strings with values (such as vari-
ables and expressions) and this syntax isn’t sufficient, you can use the .(dot) oper-
ator to concatenate two or more strings. This operator is covered in a later section.
\n Newline.
\t Tab.
\" Double quote.
\\ Backslash.
\0 ASCII 0 (null).
\r Line feed.
\$ Escape $sign so that it is not treated as a variable but as the
character $.
\{Octal #} The character represented by the specified octal #—for exam-
ple, \70represents the letter 8.
\x{Hexadecimal #} The character represented by the specified hexadecimal #—for
example, \0x32represents the letter 2.
2.5.3.2 Single Quotes In addition to double quotes, single quotes m
delimit strings. However, in contrast to double quotes, single quotes
support all the double quotes’ escaping and variable substitution.
The following table includes the only two escapings supported by
quotes:
Newline.
Tab.
Double quote.
Backslash.
ASCII 0 (null).
Line feed.
Escape $sign so that it is not treated as a variable
character $.
tal #} The character represented by the specified octal #—
ple, \70represents the letter 8.
exadecimal #} The character represented by the specified hexadec
example, \0x32represents the letter 2.
Single quote.
Backslash, used when wanting to represent a back
lowed by a single quote—for example, \\'.Examples:
'Hello, World'
'Today\'s the day'
2.5.3.3 Here-Docs Here-docsenable you to embed large pieces of text in
your scripts, which may include lots of double quotes and single quotes, with-out having to constantly escape them.
The following is an example of a here-doc:
<<
The acronym "PHP" is therefore, usually referred to as a recursive acronym
➥because the long form contains the acronym itself.
As this text is being written in a here-doc there is no need to escape the
➥double quotes.
THE_END
The strings starts with <<<, followed by a string that you know doesn’t
appear in your text. It is terminated by writing that string at the beginning of
a line, followed by an optional semicolon (;), and then a required newline (\n).
Escaping and variable substitution in here-docs is identical to double-quoted
strings except that you are not required to escape double quotes.
2.5.3.4 Accessing String Offsets Individual characters in a string can be
accessed using the $str{offset}notation. You can use it to both read and write
string offsets. When reading characters, this notation should be used only to
access valid indices. When modifying characters, you may access offsets that
don’t yet exist. PHP automatically sets that offset to the said character, and if
this results in a gap between the ending of the original string and the offset of
the new character, the gap filled with space characters (' ').
This example creates and prints the string "Andi"(in an awkward way):
$str = "A";
$str{2} = "d";
$str{1} = "n";
$str = $str . "i";
print $str;
Tip: For many cases, PHP has string manipulation functions which use effi-cient algorithms. You should first look at them before you access strings
directly using string offsets. They are usually prefixed with str_. For more
complex needs, the regular expressions functions—most notably the pcre_
family of functions—will come in handyThis support still exists in PHP 5, and you are likely to bump into it often.
However, you should really use the {}notation because it differentiates string
offsets from array offsets and thus, makes your code more readable.
2.5.4 Booleans
Booleans were introduced for the first time in PHP 4 and didn’t exist in prior
versions. A Boolean value can be either trueor false.
As previously mentioned, PHP automatically converts types when
needed. Boolean is probably the type that other types are most often converted
to behind the scenes. This is because, in any conditional code such as if state-ments, loops, and so on, types are converted to this scalar type to check if the
condition is satisfied. Also, comparison operators result in a Boolean value.
Consider the following code fragment:
$numerator = 1;
$denominator = 5;
if ($denominator == 0) {
print "The denominator needs to be a non-zero number\n";
}
The result of the equal-than operator is a Boolean; in this case, it would
be falseand, therefore, the if() statement would not be entered.
Now, consider the next code fragment:
$numerator = 1;
$denominator = 5;
if ($denominator) {
/* Perform calculation */
} else {
print "The denominator needs to be a non-zero number\n";
}
You can see that no comparison operator was used in this example; how-ever, PHP automatically internally converted $denominatoror, to be more accu-rate, the value 5to its Boolean equivalent, true, to perform the if()statement
and, therefore, enter the calculation.
Although not all types have been covered yet, the following table shows
truth values for their values. You can revisit this table to check for the types of
Boolean value equivalents, as you learn about the remaining types.2.5.5 Null
Nullis a data type with only one possible value: the NULLvalue. It marks vari-
ables as being empty, and it’s especially useful to differentiate between the
empty string and null values of databases.
The isset($variable)operator of PHP returns falsefor NULL, and truefor
any other data type, as long as the variable you’re testing exists.
The following is an example of using NULL:
$value = NULL;
2.5.6 Resources
Resources, a special data type, represent a PHP extension resource such as a
database query, an open file, a database connection, and lots of other external
types.
You will never directly touch variables of this type, but will pass them
around to the relevant functions that know how to interact with the specified
resource.
2.5.7 Arrays
An arrayin PHP is a collection of key/value pairs. This means that it maps
keys (or indexes) to values. Array indexescan be either integers or strings
whereas values can be of any type (including other arrays).
Tip:Arrays in PHP are implemented using hash tables, which means that
accessing a value has an average complexity of O(1).
2.5.7.1array()construct Arrays can be declared using the array()lan-
guage construct, which generally takes the following form (elements inside
square brackets, [], are optional):
array([key =>] value, [key =>] value, ...)
Data Type False Values True Values
Integer 0 All non-zero values
Floating point 0.0 All non-zero values
Strings Empty strings ()""
The zero string ()"0"
All other strings
Null Always Never
Array If it does not contain
any elements
If it contains at least
one element
Object Never Always
Resource Never AlwaysThe key is optional, and when it’s not specified, the key is automatically
assigned one more than the largest previous integer key (starting with 0). You
can intermix the use with and without the key even within the same declara-tion.
The value itself can be of any PHP type, including an array. Arrays con-taining arrays give a similar result as multi-dimensional arrays in other lan-guages.
Here are a few examples:
☞array(1, 2, 3)is the same as the more explicit array(0 => 1, 1 => 2, 2
➥=> 3).
☞array("name" => "John", "age" => 28)
☞array(1 => "ONE", "TWO", "THREE")is equivalent to array(1 => "ONE", 2 =>
➥"TWO", 3 => "THREE").
☞array() an empty array.
Here’s an example of a nested array()statement:
array(array("name" => "John", "age" => 28), array("name" =>
➥"Barbara", "age" => 67))
The previous example demonstrates an array with two elements: Each
one is a collection (array) of a person’s information.
2.5.7.2 Accessing Array Elements Array elements can be accessed by using
the $arr[key]notation, where keyis either an integer or string expression.
When using a constant string for key,make sure you don’t forget the single or
double quotes, such as $arr["key"]. This notation can be used for both reading
array elements and modifying or creating new elements.
2.5.7.3 Modifying/Creating Array Elements
$arr1 = array(1, 2, 3);
$arr2[0] = 1;
$arr2[1] = 2;
$arr2[2] = 3;
print_r($arr1);
print_r($arr2);
The print_r()function has not been covered yet in this book, but when it
is passed an array, it prints out the array’s contents in a readable way. You can
use this function when debugging your scripts.
The previous example prints
Array
(
[0] => 1[1] => 2
[2] => 3
)
Array
(
[0] => 1
[1] => 2
[2] => 3
)
So, you can see that you can use both the array()construct and the
$arr[key] notation to create arrays. Usually, array()is used to declare arrays
whose elements are known at compile-time, and the $arr[key]notation is used
when the elements are only computed at runtime.
PHP also supports a special notation, $arr[], where the key is not speci-
fied. When creating new array offsets using this notation (fo example, using it
as the l-value), the key is automatically assigned as one more than the largest
previous integer key.
Therefore, the previous example can be rewritten as follows:
$arr1 = array(1, 2, 3);
$arr2[] = 1;
$arr2[] = 2;
$arr2[] = 3;
The result is the same as in the previous example.
The same holds true for arrays with string keys:
$arr1 = array("name" => "John", "age" => 28);
$arr2["name"] = "John";
$arr2["age"] = 28;
if ($arr1 == $arr2) {
print '$arr1 and $arr2 are the same' . "\n";
}
The message confirming the equality of both arrays is printed.
2.5.7.4 Reading array values You can use the $arr[key]notation to read
array values. The next few examples build on top of the previous example:
print $arr2["name"];
if ($arr2["age"] < 35) {
print " is quite young\n";
}This example prints
John is quite young
Note:As previously mentioned, using the $arr[]syntax is not supported
when reading array indexes, but only when writing them.
2.5.7.5 Accessing Nested Arrays (or Multi-Dimensional Arrays) When
accessing nested arrays, you can just add as many square brackets as required
to reach the relevant value. The following is an example of how you can
declare nested arrays:
$arr = array(1 => array("name" => "John", "age" => 28), array("name"
➥=> "Barbara", "age" => 67))
You could achieve the same result with the following statements:
$arr[1]["name"] = "John";
$arr[1]["age"] = 28;
$arr[2]["name"] = "Barbara";
$arr[2]["age"] = 67;
Reading a nested array value is trivial using the same notation. For
example, if you want to print John’s age, the following statement does the
trick:
print $arr[1]["age"];
2.5.7.6 Traversing Arrays Using foreach There are a few different ways of
iterating over an array. The most elegant way is the foreach()loop construct.
The general syntax of this loop is
foreach($array as [$key =>] [&] $value)
...
$keyis optional, and when specified, it contains the currently iterated
value’s key, which can be either an integer or a string value, depending on the
key’s type.
Specifying &for the value is also optional, and it has to be done if you are
planning to modify $valueand want it to propagate to $array.In most cases,
you won’t want to modify the $valuewhen iterating over an array and will,
therefore, not need to specify it.Here’s a short example of the foreach()loop:
$players = array("John", "Barbara", "Bill", "Nancy");
print "The players are:\n";
foreach ($players as $key => $value) {
print "#$key = $value\n";
}
The output of this example is
The players are:
#0 = John
#1 = Barbara
#2 = Bill
#3 = Nancy
Here’s a more complicated example that iterates over an array of people
and marks which person is considered old and which one is considered young:
$people = array(1 => array("name" => "John", "age" => 28),
➥array("name" => "Barbara", "age" => 67));
foreach ($people as &$person) {
if ($person["age"] >= 35) {
$person["age group"] = "Old";
} else {
$person["age group"] = "Young";
}
}
print_r($people);
Again, this code makes use of the print_r()function.
The output of the previous code is the following:
Array
(
[1] => Array
(
[name] => John
[age] => 28
[age group] => Young
)
[2] => Array
(
[name] => Barbara
[age] => 67
[age group] => Old)
)
You can see that both the John and Barbara arrays inside the $people
array were added an additional value with their respective age group.
2.5.7.7 Traversing Arrays Using list()and each() Although foreach()
is the nicer way of iterating over an array, an additional way of traversing an
array is by using a combination of the list()construct and the each()func-
tion:
$players = array("John", "Barbara", "Bill", "Nancy");
reset($players);
while (list($key, $val) = each($players)) {
print "#$key = $val\n";
}
The output of this example is
#0 = John
#1 = Barbara
#2 = Bill
#3 = Nancy
2.5.7.8reset() Iteration in PHP is done by using an internal array pointer
that keeps record of the current position of the traversal. Unlike with
foreach(), when you want to use each()to iterate over an array, you must
reset()the array before you start to iterate over it. In general, it is best for
you to always use foreach()and not deal with this subtle nuisance of each()
traversal.
2.5.7.9each() The each()function returns the current key/value pair and
advances the internal pointer to the next element. When it reaches the end
of of the array, it returns a booloean value of false. The key/value pair is
returned as an array with four elements: the elements 0and "key", which
have the value of the key, and elements 1and "value", which have the value
of the value. The reason for duplication is that, if you’re accessing these ele-
ments individually, you’ll probably want to use the names such as
$elem["key"]and $elem["value"]:
$ages = array("John" => 28, "Barbara" => 67);
reset($ages);
$person = each($ages);print $person["key"];
print " is of age ";
print $person["value"];
This prints
John is of age 28
When we explain how the list()construct works, you will understand
why offsets 0 and 1 also exist.
2.5.7.10list() The list()construct is a way of assigning multiple array
offsets to multiple variables in one statement:
list($var1, $var2, ...) = $array;
The first variable in the list is assigned the array value at offset 0, the
second is assigned offset 1, and so on. Therefore, the list()construct trans-lates into the following series of PHP statements:
$var1 = $array[0];
$var2 = $array[1];
...
As previously mentioned, the indexes 0 and 1 returned by each()are
used by the list()construct. You can probably already guess how the combi-nation of list()and each()work.
Consider the highlighted line from the previous $playerstraversal example:
$players = array("John", "Barbara", "Bill", "Nancy");
reset($players);
while (list($key, $val) = each($players)) {
print "#$key = $val\n";
}
What happens in the boldfaced line is that during every loop iteration,
each()returns the current position’s key/value pair array, which, when exam-ined with print_r(), is the following array:
Array
(
[1] => John
[value] => John[0] => 0
[key] => 0
)
Then, the list()construct assigns the array’s offset 0 to $keyand offset 1
to $val.
2.5.7.11 Additional Methods for Traversing Arrays You can use other func-tions to iterate over arrays including current()and next(). You shouldn’t use
them because they are confusing and are legacy functions. In addition, some
standard functions allow all sorts of elegant ways of dealing with arrays such
as array_walk(), which is covered in a later chapter.
2.5.8 Constants
In PHP, you can define names, called constants, for simple values. As the
name implies, you cannot change these constants once they represent a cer-tain value. The names for constants have the same rules as PHP variables
except that they don’t have the leading dollar sign. It is common practice in
many programming languages—including PHP—to use uppercase letters for
constant names, although you don’t have to. If you wish, which we do not rec-ommend, you may define your constants as case-insensitive, thus not requir-ing code to use the correct casing when referring to your constants.
Tip:Only use case-sensitive constants both to be consistent with accepted cod-ing standards and because it is unclear if case-insensitive constants will con-tinued to be supported in future versions of PHP.
Unlike variables, constants, once defined, are globally accessible. You
don’t have to (and can’t) redeclare them in each new function and PHP file.
To define a constant, use the following function:
define("CONSTANT_NAME", value [, case_sensitivity])
Where:
☞"CONSTANT_NAME"is a string.
☞valueis any valid PHP expression excluding arrays and objects.
☞case_sensitivityis a Boolean (true/false) and is optional. The default is
true.
An example for a built-in constant is the Boolean value true, which is
registered as case-insensitive.
Here’s a simple example for defining and using a constant:
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