PHP's handling of objects has been completely rewritten, allowing for better performance and more features. In previous versions of PHP, objects were handled like primitive types (for instance integers and strings). The drawback of this method was that semantically the whole object was copied when a variable was assigned, or pass as a parameter to a method. In the new approach, objects are referenced by handle, and not by value (one can think of a handle as an object's identifier).
Many PHP programmers aren't even aware of the copying quirks of the old object model and, therefore, the majority of PHP applications will work out of the box, or with very few modifications.
Private and Protected MembersPHP 5 introduces private and protected member variables, they allow do define the visibility of class properties.
ExampleProtected member variables can be accessed in classes extending the class they are declared in, whereas private member variables can only be accessed by the class they belong to.
<?php
class MyClass {
private $Hello = "Hello, World!\n";
protected $Bar = "Hello, Foo!\n";
protected $Foo = "Hello, Bar!\n";
function printHello() {
print "MyClass::printHello() " . $this->Hello;
print "MyClass::printHello() " . $this->Bar;
print "MyClass::printHello() " . $this->Foo;
}
}
class MyClass2 extends MyClass {
protected $Foo;
function printHello() {
MyClass::printHello(); /* Should print */
print "MyClass2::printHello() " . $this->Hello; /* Shouldn't print out anything */
print "MyClass2::printHello() " . $this->Bar; /* Shouldn't print (not declared)*/
print "MyClass2::printHello() " . $this->Foo; /* Should print */
}
}
$obj = new MyClass();
print $obj->Hello; /* Shouldn't print out anything */
print $obj->Bar; /* Shouldn't print out anything */
print $obj->Foo; /* Shouldn't print out anything */
$obj->printHello(); /* Should print */
$obj = new MyClass2();
print $obj->Hello; /* Shouldn't print out anything */
print $obj->Bar; /* Shouldn't print out anything */
print $obj->Foo; /* Shouldn't print out anything */
$obj->printHello();
?>
With PHP 5 (Zend Engine 2), private and protected methods are also introduced.
Example<?php
class Foo {
private function aPrivateMethod() {
echo "Foo::aPrivateMethod() called.\n";
}
protected function aProtectedMethod() {
echo "Foo::aProtectedMethod() called.\n";
$this->aPrivateMethod();
}
}
class Bar extends Foo {
public function aPublicMethod() {
echo "Bar::aPublicMethod() called.\n";
$this->aProtectedMethod();
}
}
$o = new Bar;
$o->aPublicMethod();
?>
Old code that has no user-defined classes or functions named "public," "protected" or "private" should run without modifications.
Abstract Classes and MethodsPHP 5 also introduces abstract classes and methods. An abstract method only declares the method's signature and does not provide an implementation. A class that contains abstract methods needs to be declared abstract.
Example<?php
abstract class AbstractClass {
abstract public function test();
}
class ImplementedClass extends AbstractClass {
public function test() {
echo "ImplementedClass::test() called.\n";
}
}
$o = new ImplementedClass;
$o->test();
?>
Abstract classes cannot be instantiated. Old code that has no user-defined classes or functions named 'abstract' should run without modifications.
InterfacesThe Zend Engine 2.0 introduces interfaces. A class may implement an arbitrary list of interfaces.
Example<?php
interface Throwable {
public function getMessage();
}
class Exception implements Throwable {
public function getMessage() {
// ...
}
?>
Old code that has no user-defined classes or functions named 'interface' or 'implements' should run without modifications.
Class Type HintsWhile remaining loosely typed PHP 5 introduces the ability to use class type hints to declare the expected class of objects that are passed as parameters to a method.
Example<?php
interface Foo {
function a(Foo $foo);
}
interface Bar {
function b(Bar $bar);
}
class FooBar implements Foo, Bar {
function a(Foo $foo) {
// ...
}
function b(Bar $bar) {
// ...
}
}
$a = new FooBar;
$b = new FooBar;
$a->a($b);
$a->b($b);
?>
These class type hints are not checked upon compilation, as would be the case in a typed language, but during runtime. This means that:
<?php
function foo(ClassName $object) {
// ...
}
?>
is equivalent to:
<?php
function foo($object) {
if (!($object instanceof ClassName)) {
die("Argument 1 must be an instance of ClassName");
}
}
?>
This syntax only applies to objects/classes, not built-in types.
finalPHP 5 introduces the "final" keyword to declare final members and methods. Methods and members declared final cannot be overridden by sub-classes.
Example<?php
class Foo {
final function bar() {
// ...
}
}
?>
Old code that has no user-defined classes or functions named 'final' should run without modifications.
Object CloningPHP 4 (Zend Engine 1.0) offered no way a user could decide what copy constructor to run when an object is duplicated. During duplication, PHP 4 did a bit for bit copy making an identical replica of all the object's properties.
Creating a copy of an object with fully replicated properties is not always the wanted behavior. A good example of the need for copy constructors, is if you have an object which represents a GTK window and the object holds the resource of this GTK window, when you create a duplicate you might want to create a new window with the same properties and have the new object hold the resource of the new window. Another example is if your object holds a reference to another object which it uses and when you replicate the parent object you want to create a new instance of this other object so that the replica has its own separate copy.
An object copy is created by calling the object's __clone() method:
<?php
$copy_of_object = $object->__clone();
?>
When the developer asks to create a new copy of an object, the Zend Engine will check if a __clone() method has been defined or not. If not, it will call a default __clone() which will copy all of the object's properties. If a __clone() method is defined, then it will be responsible to set the necessary properties in the created object. For convenience, the engine will supply a function that imports all of the properties from the source object, so that they can start with a by-value replica of the source object, and only override properties that need to be changed.
Example<?php
class MyCloneable {
static $id = 0;
function MyCloneable() {
$this->id = self::$id++;
}
function __clone() {
$this->name = $that->name;
$this->address = "New York";
$this->id = self::$id++;
}
}
$obj = new MyCloneable();
$obj->name = "Hello";
$obj->address = "Tel-Aviv";
print $obj->id . "\n";
$obj = $obj->__clone();
print $obj->id . "\n";
print $obj->name . "\n";
print $obj->address . "\n";
?>
The Zend Engine allows developers to declare constructor methods for classes. Classes which have a constructor method call this method on each newly-created object, so it is suitable for any initialization that the object may need before it is used.
With PHP 4, constructor methods were class methods that had the same name as the class itself. Since it is very common to call parent constructors from derived classes, the way PHP 4 worked made it a bit cumbersome to move classes around in a large class hierarchy. If a class is moved to reside under a different parent, the constructor name of that parent changes as well, and the code in the derived class that calls the parent constructor has to be modified.
PHP 5 introduces a standard way of declaring constructor methods by calling them by the name __construct().
Example<?php
class BaseClass {
function __construct() {
print "In BaseClass constructor\n";
}
}
class SubClass extends BaseClass {
function __construct() {
parent::__construct();
print "In SubClass constructor\n";
}
}
$obj = new BaseClass();
$obj = new SubClass();
?>
For backwards compatibility, if PHP 5 cannot find a __construct() function for a given class, it will search for the old-style constructor function, by the name of the class. Effectively, it means that the only case that would have compatibility issues is if the class had a method named __construct() which was used for different semantics.
DestructorsHaving the ability to define destructors for objects can be very useful. Destructors can log messages for debugging, close database connections and do other clean-up work. No mechanism for object destructors existed in PHP 4, although PHP had already support for registering functions which should be run on request shutdown.
PHP 5 introduces a destructor concept similar to that of other object-oriented languages, such as Java: When the last reference to an object is destroyed the object's destructor, which is a class method name %__destruct()% that recieves no parameters, is called before the object is freed from memory.
Example<?php
class MyDestructableClass {
function __construct() {
print "In constructor\n";
$this->name = "MyDestructableClass";
}
function __destruct() {
print "Destroying " . $this->name . "\n";
}
}
$obj = new MyDestructableClass();
?>
Like constructors, parent destructors will not be called implicitly by the engine. In order to run a parent destructor, one would have to explicitly call parent::__destruct() in the destructor body.
ConstantsPHP 5 introduces per-class constants:
<?php
class Foo {
const constant = "constant";
}
echo "Foo::constant = " . Foo::constant . "\n";
?>
PHP 5 allows for expressions within constants, however, constants are evaluated at compile time, therefore no constants can be declared that rely on runtime information.
<?php
class Bar {
const a = 1<<0;
const b = 1<<1;
const c = a | b;
}
?>
Old code that has no user-defined classes or functions named 'const' will run without modifications.
ExceptionsPHP 4 had no exception handling. PHP 5 introduces a exception model similar to that of other programming languages.
Example<?php
class MyExceptionFoo extends Exception {
function __construct($exception) {
parent::__construct($exception);
}
}
try {
throw new MyExceptionFoo("Hello");
} catch (MyException $exception) {
print $exception->getMessage();
}
?>
Old code that has no user-defined classes or functions 'catch', 'throw' and 'try' will run without modifications.
Dereferencing objects returned from functionsIn PHP 4 it wasn't possible to dereference objects returned by functions and make further method calls on those objects. With the advent of Zend Engine 2, the following is now possible:
<?php
class Circle {
function draw() {
print "Circle\n";
}
}
class Square {
function draw() {
print "Square\n";
}
}
function ShapeFactoryMethod($shape) {
switch ($shape) {
case "Circle":
return new Circle();
case "Square":
return new Square();
}
}
ShapeFactoryMethod("Circle")->draw();
ShapeFactoryMethod("Square")->draw();
?>
<?php
class foo {
static $my_static = 5;
}
print foo::$my_static;
?>
PHP 5 introduces the 'static' keyword to declare a method static, thus callable from outside the object context.
Example<?php
class Foo {
public static function aStaticMethod() {
// ...
}
}
Foo::aStaticMethod();
?>
The pseudo variable $this is not available inside a method that has been declared static.
instanceofPHP 5 introduces the instanceof keyword, that allows you to ascertain whether or not an object is an instance of a class, or extends a class, or implements an interface.
Example<?php
class baseClass { }
$a = new baseClass;
if ($a instanceof basicClass) {
echo "Hello World";
}
?>
Statics are now treated at compile-time which allows developers to assign variables to statics by reference. This change also greatly improves their performance but means that indirect references to statics will not work anymore.
Parameters that are passed by reference to a function may now have default values Example<?php
function my_function(&$var = null) {
if ($var === null) {
die("$var needs to have a value");
}
}
?>
The __autoload() interceptor function will be automatically called when an undeclared class is to be instantiated. The name of that class will be passed to the __autoload() interceptor function as its only argument.
Example<?php
function __autoload($className) {
include_once $className . ".php";
}
$object = new ClassName;
?>
Both method calls and property accesses can be overloaded via the __call(), __get() and __set() methods.
Example: __get() and __set()<?php
class Setter {
public $n;
public $x = array("a" => 1, "b" => 2, "c" => 3);
function __get($nm) {
print "Getting [$nm]\n";
if (isset($this->x[$nm])) {
$r = $this->x[$nm];
print "Returning: $r\n";
return $r;
} else {
print "Nothing!\n";
}
}
function __set($nm, $val) {
print "Setting [$nm] to $val\n";
if (isset($this->x[$nm])) {
$this->x[$nm] = $val;
print "OK!\n";
} else {
print "Not OK!\n";
}
}
}
$foo = new Setter();
$foo->n = 1;
$foo->a = 100;
$foo->a++;
$foo->z++;
var_dump($foo);
?>
<?php
class Caller {
var $x = array(1, 2, 3);
function __call($m, $a) {
print "Method $m called:\n";
var_dump($a);
return $this->x;
}
}
$foo = new Caller();
$a = $foo->test(1, "2", 3.4, true);
var_dump($a);
?>
--------------------------
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