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<title>Scope Resolution Operator (::)</title>
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<h2 class="title">Scope Resolution Operator (<em>::</em>)</h2>
<div class="caution"><strong class="caution">Caution</strong>
<p class="simpara">
The following is valid for PHP 4 and later only.
</p>
</div>
<p class="para">
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 <em>::</em> operator
is being used for this.
</p>
<div class="informalexample">
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">A </span><span style="color: #007700">{<br /> function </span><span style="color: #0000BB">example</span><span style="color: #007700">() {<br /> echo </span><span style="color: #DD0000">"I am the original function A::example().<br />\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br />class </span><span style="color: #0000BB">B </span><span style="color: #007700">extends </span><span style="color: #0000BB">A </span><span style="color: #007700">{<br /> function </span><span style="color: #0000BB">example</span><span style="color: #007700">() {<br /> echo </span><span style="color: #DD0000">"I am the redefined function B::example().<br />\n"</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">A</span><span style="color: #007700">::</span><span style="color: #0000BB">example</span><span style="color: #007700">();<br /> }<br />}<br /><br /></span><span style="color: #FF8000">// there is no object of class A.<br />// this will print<br />// I am the original function A::example().<br /><br /></span><span style="color: #0000BB">A</span><span style="color: #007700">::</span><span style="color: #0000BB">example</span><span style="color: #007700">();<br /><br /></span><span style="color: #FF8000">// create an object of class B.<br /></span><span style="color: #0000BB">$b </span><span style="color: #007700">= new </span><span style="color: #0000BB">B</span><span style="color: #007700">;<br /><br /></span><span style="color: #FF8000">// this will print <br />// I am the redefined function B::example().<br /><br />// I am the original function A::example().<br /><br /></span><span style="color: #0000BB">$b</span><span style="color: #007700">-></span><span style="color: #0000BB">example</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="para">
The above example calls the function <em>example()</em> in
class A, but there is no object of class A, so that
we cannot write <em>$a->example()</em> or similar. Instead we
call <em>example()</em> as a 'class function', that is, as a
function of the class itself, not any object of that
class.
</p>
<p class="para">
There are class functions, but there are no class variables.
In fact, there is no object at all at the time of the call.
Thus, a class function may not use any object variables (but
it can use local and global variables), and it may not use
<var class="varname"><var class="varname">$this</var></var> at all.
</p>
<p class="para">
In the above example, class B redefines the function
<em>example()</em>. The original definition in class A is shadowed
and no longer available, unless you are referring specifically
to the implementation of <em>example()</em> in class A using the
::-operator. Write <em>A::example()</em> to do this (in fact, you
should be writing <em>parent::example()</em>, as shown in the next
section).
</p>
<p class="para">
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"><var class="varname">$this</var></var> and object variables.
</p>
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