PHP 5

1  PHP 5

1.1 INTRODUCTION
                 Only time will tell if the PHP 5 release will be as successful as its two prede- cessors (PHP 3 and PHP 4). The new features and changes aim to rid PHP of any weaknesses it may have had and make sure that it stays in the lead as the world’s best web-scripting language. This book details PHP 5 and its new features. However, if you are familiar with PHP 4 and are eager to know what is new in PHP 5, this chapter is for you. When you finish reading this chapter, you will have learned ☞The new language features ☞News concerning PHP extensions ☞Other noteworthy changes to PHP’s latest version  
1.2  LANGUAGE FEATURES 
1.2.1 New Object-Oriented Model 
When Zeev Suraski added the object-oriented syntax back in the days of PHP 3, it was added as “syntactic sugar for accessing collections.” The OO model also had support for inheritance and allowed a class (and object) to aggregate both methods and properties, but not much more. When Zeev and Andi Gut- mans rewrote the scripting engine for PHP 4, it was a completely new engine; it ran much faster, was more stable, and boasted more features. However, the OO model first introduced in PHP 3 was barely touched. Although the object model had serious limitations, it was used exten- sively around the world, often in large PHP applications. This impressive use of the OOP paradigm with PHP 4, despite its weaknesses, led to it being the main focus for the PHP 5 release.So, what were some of the limitations in PHP 3 and 4? The biggest limi-tation (which led to further limitations) was the fact that the copy semantics of
objects were the same as for native types. So, how did this actually affect the
PHP developer? When assigning a variable (that points to an object) to
another variable, a copy of the object would be created. Not only did this
impact performance, but it also usually led to obscure behavior and bugs in
PHP 4 applications because many developers thought that both variables
would point at the same object, which was not the case. The variables were
instead pointing at separate copies of the same object. Changing one would
not change the other.
For example:
class Person {
var $name;
function getName()
{
return $this->name;
}
function setName($name)
{
$this->name = $name;
}
function Person($name)
{
$this->setName($name);
}
}
function changeName($person, $name)
{
$person->setName($name);
}
$person = new Person("Andi");
changeName($person, "Stig");
print $person->getName();
In PHP 4, this code would print out "Andi". The reason is that we pass
the object $personto the changeName()function by-value, and thus, $personis
copied and changeName()works on a copy of $person.
This behavior is not intuitive, as many developers would expect the Java-like behavior. In Java, variables actually hold a handle (or pointer) to the
object, and therefore, when it is copied, only the handle (and not the entire
object) is duplicated.
There were two kinds of users in PHP 4: the ones who were aware of this
problem and the ones who were not. The latter would usually not notice this
problem and their code was written in a way where it did not really matter if
the problem existed. Surely some of these people had sleepless nights trying to
track down weird bugs that they could not pinpoint. The former group dealt
with this problem by always passing and assigning objects by reference. This
would prevent the engine from copying their objects, but it would be a head-ache because the code included numerous & signs.The old object model not only led to the afore-mentioned problems, but
also to fundamental problems that prevented implementing some additional
features on top of the existing object model.
In PHP 5, the infrastructure of the object model was rewritten to work
with object handles. Unless you explicitly clone an object by using the clone
keyword, you never create behind-the-scenes duplicates of your objects. In
PHP 5, you don’t need a need to pass objects by reference or assign them by
reference.
Note:Passing by reference and assigning by reference are still sup-ported, in case you want to actually change a variable’s content (whether
object or other type).1.2.2 New Object-Oriented Features
The new OO features are too numerous to give a detailed description in this
section. Chapter 3, “PHP 5 OO Language,” details each feature.
The following list provides the main new features:
☞public/private/protectedaccess modifiers for methods and properties.
Allows the use of common OO access modifiers to control access to
methods and properties:
class MyClass {
private $id = 18;
public function getId() {
return $this->id;
}
}
☞Unified constructor name __construct().
Instead of the constructor being the name of the class, it is now declared
as __construct(), which makes it easier to shift classes inside class hier-archies:
class MyClass {
function __construct() {
print "Inside constructor";
}
}
☞Object destructor support by defining a __destructor() method.
Allows defining a destructor function that runs when an object
is destroyed:
class MyClass {
function __destruct() {
print ”Destroying object”;
}
}
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4  What Is New in PHP 5? Chap. 1
☞Interfaces.
Gives the ability for a class to fulfill more than one is-a relationships. A class can
inherit only from one class, but may implement as many interfaces as it wants:
interface Display {
function display();
}
class Circle implements Display {
function display() {
print "Displaying circle\n";
}
}
☞instanceofoperator.
Language-level support for is-a relationship checking. The PHP 4 is_a() function
is now deprecated:
if ($obj instanceof Circle) {
print '$obj is a Circle';
}
☞Final methods.
The finalkeyword allows you to mark methods so that an inheriting class cannot overload
them:
class MyClass {
final function getBaseClassName() {
return __CLASS__;
}
}
☞Final classes.
After declaring a class as final, it cannot be inherited. The following example
would error out.
final class FinalClass {
}
class BogusClass extends FinalClass {
}
☞Explicit object cloning.
To clone an object, you must use the clonekeyword. You may declare a __clone()
method, which will be called during the clone process (after the properties have
been copied from the original object):
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class MyClass {
function __clone() {
print "Object is being cloned";
}
}
$obj = new MyClass();
$obj_copy = clone $obj;
☞Class constants.
Class definitions can now include constant values and are referenced
using the class:
class MyClass {
const SUCCESS = "Success";
const FAILURE = "Failure";
}
print MyClass::SUCCESS;
☞Static methods.
You can now define methods as static by allowing them to be called from
non-object context. Static methods do not define the $thisvariable
because they are not bound to any specific object:
class MyClass {
static function helloWorld() {
print "Hello, world";
}
}
MyClass::helloWorld();
☞Static members.
Class definitions can now include static members (properties) that are
accessible via the class. Common usage of static members is in the
Singleton pattern:
class Singleton {
static private $instance = NULL;
private function __construct() {
}
static public function getInstance() {
if (self::$instance == NULL) {
self::$instance = new Singleton();
}
return self::$instance;
}
}
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6  What Is New in PHP 5? Chap. 1
☞Abstract classes.
A class may be declared abstractto prevent it from being instantiated.
However, you may inherit from an abstract class:
abstract class MyBaseClass {
function display() {
print "Default display routine being called";
}
}
☞Abstract methods.
A method may be declared abstract, thereby deferring its definition to an
inheriting class. A class that includes abstract methods must be declared
abstract:
abstract class MyBaseClass {
abstract function display();
}
☞Class type hints.
Function declarations may include class type hints for their parameters.
If the functions are called with an incorrect class type, an error occurs:
function expectsMyClass(MyClass $obj) {
}
☞Support for dereferencing objects that are returned from methods.
In PHP 4, you could not directly dereference objects that were returned
from methods. You had to first assign the object to a dummy variable and
then dereference it.
PHP 4:
$dummy = $obj->method();
$dummy->method2();
PHP 5:
$obj->method()->method2();
☞Iterators.
PHP 5 allows both PHP classes and PHP extension classes to implement
an Iteratorinterface. After you implement this interface, you can iterate
instances of the class by using the foreach()language
construct:
$obj = new MyIteratorImplementation();
foreach ($obj as $value) {
print "$value";
}
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