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>Chapter 14. Classes and Objects</TD
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><H1
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><TT
CLASS="literal"
>Constructors</TT
></H1
><DIV
CLASS="caution"
><P
></P
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><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
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ALIGN="LEFT"
><P
>&#13;     In PHP 3 and PHP 4 constructors behave differently. The PHP 4
     semantics are strongly preferred.
    </P
></TD
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></DIV
><P
>&#13;    Constructors are functions in a class that are automatically
    called when you create a new instance of a class with
    <TT
CLASS="literal"
>new</TT
>. 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).
   </P
><DIV
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><A
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><P
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><TR
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><PRE
CLASS="php"
>// Works in PHP 3 and PHP 4.
class Auto_Cart extends Cart
{
    function Auto_Cart()
    {
        $this-&#62;add_item ("10", 1);
    }
}</PRE
></TD
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><P
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><P
>&#13;    This defines a class Auto_Cart that is a Cart plus a constructor
    which initializes the cart with one item of article number "10"
    each time a new Auto_Cart is being made with "new". Constructors
    can take arguments and these arguments can be optional, which
    makes them much more useful. To be able to still use the class
    without parameters, all parameters to constructors should be
    made optional by providing default values.
   </P
><DIV
CLASS="informalexample"
><A
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></A
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><PRE
CLASS="php"
>// Works in PHP 3 and PHP 4.
class Constructor_Cart extends Cart
{
    function Constructor_Cart($item = "10", $num = 1)
    {
        $this-&#62;add_item ($item, $num);
    }
}
 
// Shop the same old boring stuff.
 
$default_cart = new Constructor_Cart;
 
// Shop for real...
 
$different_cart = new Constructor_Cart("20", 17);</PRE
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><P
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><P
>&#13;    You also can use the <TT
CLASS="literal"
>@</TT
> operator to
    <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>mute</I
></SPAN
> errors occuring in the constructor, e.g.
    <TT
CLASS="literal"
>@new</TT
>.
   </P
><DIV
CLASS="caution"
><P
></P
><TABLE
CLASS="caution"
BORDER="1"
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><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
>&#13;     In PHP 3, derived classes and constructors have a number of
     limitations. The following examples should be read carefully
     to understand these limitations.
    </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="informalexample"
><A
NAME="AEN5500"
></A
><P
></P
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><PRE
CLASS="php"
>class A
{
    function A()
    {
      echo "I am the constructor of A.&#60;br&#62;\n";
    }
}

class B extends A
{
    function C()
    {
        echo "I am a regular function.&#60;br&#62;\n";
    }
}

// no constructor is being called in PHP 3.
$b = new B;</PRE
></TD
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><P
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></DIV
><P
>&#13;    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.
   </P
><P
>&#13;    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.&#60;br&#62;' in PHP 4.
   </P
><DIV
CLASS="informalexample"
><A
NAME="AEN5504"
></A
><P
></P
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><PRE
CLASS="php"
>class A
{
    function A()
    {
        echo "I am the constructor of A.&#60;br&#62;\n";
    }

    function B()
    {
        echo "I am a regular function named B in class A.&#60;br&#62;\n";
        echo "I am not a constructor in A.&#60;br&#62;\n";
    }
}

class B extends A
{
    function C()
    {
        echo "I am a regular function.&#60;br&#62;\n";
    }
}

// This will call B() as a constructor.
$b = new B;</PRE
></TD
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><P
></P
></DIV
><P
>&#13;    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.
   </P
><P
>&#13;    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.&#60;br&#62;'.
   </P
><DIV
CLASS="caution"
><P
></P
><TABLE
CLASS="caution"
BORDER="1"
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><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
>&#13;     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.
    </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="note"
><BLOCKQUOTE
CLASS="note"
><P
><B
>Note: </B
>
     There are no destructors in PHP 3 or PHP 4. You may use
     <A
HREF="function.register-shutdown-function.html"
><B
CLASS="function"
>register_shutdown_function()</B
></A
> instead
     to simulate most effects of destructors.
    </P
></BLOCKQUOTE
></DIV
><P
>&#13;    Destructors are functions that are called automatically
    when an object is destroyed, either with <A
HREF="function.unset.html"
><B
CLASS="function"
>unset()</B
></A
>
    or by simply going out of scope. There are no destructors
    in PHP.
   </P
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