Constructors and Destructors

Constructor

PHP 5 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.

注: Parent constructors are not called implicitly. In order to run a parent constructor, a call to parent::__construct() is required.

例子 14-1. using new unified constructors


<?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.

Destructor

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 named __destruct() that receives no parameters, is called before the object is freed from memory.

例子 14-2. Destructor 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.