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>Hoofdstuk 13. Classes and Objects (PHP 4)</TD
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> The following is valid for PHP 4 and later only.
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> Sometimes it is useful to refer to functions and variables
in base classes or to refer to functions in classes that
have not yet any instances. The :: operator is being used
for this.
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<font color="#0000BB"><?php<br /></font><font color="#007700">class </font><font color="#0000BB">A </font><font color="#007700">{<br /> function </font><font color="#0000BB">example</font><font color="#007700">() {<br /> echo </font><font color="#DD0000">"I am the original function A::example().<br />\n"</font><font color="#007700">;<br /> }<br />}<br /><br />class </font><font color="#0000BB">B </font><font color="#007700">extends </font><font color="#0000BB">A </font><font color="#007700">{<br /> function </font><font color="#0000BB">example</font><font color="#007700">() {<br /> echo </font><font color="#DD0000">"I am the redefined function B::example().<br />\n"</font><font color="#007700">;<br /> </font><font color="#0000BB">A</font><font color="#007700">::</font><font color="#0000BB">example</font><font color="#007700">();<br /> }<br />}<br /><br /></font><font color="#FF8000">// there is no object of class A.<br />// this will print<br />// I am the original function A::example().<br /><br /></font><font color="#0000BB">A</font><font color="#007700">::</font><font color="#0000BB">example</font><font color="#007700">();<br /><br /></font><font color="#FF8000">// create an object of class B.<br /></font><font color="#0000BB">$b </font><font color="#007700">= new </font><font color="#0000BB">B</font><font color="#007700">;<br /><br /></font><font color="#FF8000">// this will print <br />// I am the redefined function B::example().<br /><br />// I am the original function A::example().<br /><br /></font><font color="#0000BB">$b</font><font color="#007700">-></font><font color="#0000BB">example</font><font color="#007700">();<br /></font><font color="#0000BB">?></font>
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> The above example calls the function example() in
class A, but there is no object of class A, so that
we cannot write $a->example() or similar. Instead we
call example() as a 'class function', that is, as a
function of the class itself, not any object of that
class.
</P
><P
> There are class functions, but there are no class variables.
In fact, there is no object at all at the time of the call.
Thus, a class function may not use any object variables (but
it can use local and global variables), and it may no use
<VAR
CLASS="varname"
>$this</VAR
> at all.
</P
><P
> In the above example, class B redefines the function example().
The original definition in class A is shadowed
and no longer available, unless you are referring specifically
to the implementation of example() in class A using the
::-operator. Write A::example() to do this (in fact, you
should be writing parent::example(), as shown in the next
section).
</P
><P
> In this context, there is a current object and it may have object
variables. Thus, when used from WITHIN an object function, you may use
<VAR
CLASS="varname"
>$this</VAR
> and object variables.
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