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Constructors

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`Constructors`

Caution

In PHP 3 and PHP 4 constructors behave differently. The PHP 4 semantics are strongly preferred.

Constructors are functions in a class that are automatically called when you create a new instance of a class with new. In PHP 3, a function becomes a constructor when it has the same name as the class. In PHP 4, a function becomes a constructor, when it has the same name as the class it is defined in - the difference is subtle, but crucial (see below).


<?php
// Works in PHP 3 and PHP 4.
class Auto_Cart extends Cart {
    function Auto_Cart() {
        $this->add_item("10", 1);
    }
}
?>

This defines a class Auto_Cart that is a Cart plus a constructor which initializes the cart with one item of article number "10" each time a new Auto_Cart is being made with "new". Constructors can take arguments and these arguments can be optional, which makes them much more useful. To be able to still use the class without parameters, all parameters to constructors should be made optional by providing default values.


<?php
// Works in PHP 3 and PHP 4.
class Constructor_Cart extends Cart {
    function Constructor_Cart($item = "10", $num = 1) {
        $this->add_item ($item, $num);
    }
}
 
// Shop the same old boring stuff.
 
$default_cart = new Constructor_Cart;
 
// Shop for real...
 
$different_cart = new Constructor_Cart("20", 17);
?>

You also can use the @ operator to mute errors occurring in the constructor, e.g. @new.

Caution

In PHP 3, derived classes and constructors have a number of limitations. The following examples should be read carefully to understand these limitations.


<?php
class A {
    function A() {
      echo "I am the constructor of A.<br />\n";
    }
}

class B extends A {
    function C() {
        echo "I am a regular function.<br />\n";
    }
}

// no constructor is being called in PHP 3.
$b = new B;
?>

In PHP 3, no constructor is being called in the above example. The rule in PHP 3 is: 'A constructor is a function of the same name as the class.'. The name of the class is B, and there is no function called B() in class B. Nothing happens.

This is fixed in PHP 4 by introducing another rule: If a class has no constructor, the constructor of the base class is being called, if it exists. The above example would have printed 'I am the constructor of A.<br />' in PHP 4.


<?php
class A
{
    function A()
    {
        echo "I am the constructor of A.<br />\n";
    }

    function B()
    {
        echo "I am a regular function named B in class A.<br />\n";
        echo "I am not a constructor in A.<br />\n";
    }
}

class B extends A
{
    function C()
    {
        echo "I am a regular function.<br />\n";
    }
}

// This will call B() as a constructor.
$b = new B;
?>

In PHP 3, the function B() in class A will suddenly become a constructor in class B, although it was never intended to be. The rule in PHP 3 is: 'A constructor is a function of the same name as the class.'. PHP 3 does not care if the function is being defined in class B, or if it has been inherited.

This is fixed in PHP 4 by modifying the rule to: 'A constructor is a function of the same name as the class it is being defined in.'. Thus in PHP 4, the class B would have no constructor function of its own and the constructor of the base class would have been called, printing 'I am the constructor of A.<br />'.

Caution

Neither PHP 3 nor PHP 4 call constructors of the base class automatically from a constructor of a derived class. It is your responsibility to propagate the call to constructors upstream where appropriate.

Note: There are no destructors in PHP 3 or PHP 4. You may use register_shutdown_function() instead to simulate most effects of destructors.

Destructors are functions that are called automatically when an object is destroyed, either with unset() or by simply going out of scope. There are no destructors in PHP.

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