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</div><hr /><div id="language.oop5.late-static-bindings" class="sect1">
<h2 class="title">Late Static Bindings</h2>
<p class="para">
As of PHP 5.3.0, PHP implements a feature called late static bindings which
can be used to reference the called class in a context of static inheritance.
</p>
<p class="para">
More precisely, late static bindings work by storing the class named in the
last "non-forwarding call". In case of static method calls, this is the
class explicitly named (usually the one on the left of the
<a href="language.oop5.paamayim-nekudotayim.html" class="link"><em>::</em></a>
operator); in case of non static method calls, it is the class of the object. A
"forwarding call" is a static one that is introduced by <em>self::</em>,
<em>parent::</em>, <em>static::</em>, or, if going
up in the class hierarchy, <span class="function"><a href="function.forward-static-call.html" class="function">forward_static_call()</a></span>.
The function <span class="function"><a href="function.get-called-class.html" class="function">get_called_class()</a></span> can be used to retrieve
a string with the name of the called class and <em>static::</em>
introduces its scope.
</p>
<p class="para">
This feature was named "late static bindings" with an internal perspective in
mind. "Late binding" comes from the fact that <em>static::</em>
will not be resolved using the class where the method is defined but it will
rather be computed using runtime information.
It was also called a "static binding" as it can be used for (but is not
limited to) static method calls.
</p>
<div class="sect2" id="language.oop5.late-static-bindings.self">
<h3 class="title">Limitations of <em>self::</em></h3>
<p class="para">
Static references to the current class like <em>self::</em> or
<em>__CLASS__</em> are resolved using the class in which the
function belongs, as in where it was defined:
</p>
<div class="example" id="example-228">
<p><strong>Example #1 <em>self::</em> usage</strong></p>
<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 /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">__CLASS__</span><span style="color: #007700">;<br /> }<br /> public static function </span><span style="color: #0000BB">test</span><span style="color: #007700">() {<br /> </span><span style="color: #0000BB">self</span><span style="color: #007700">::</span><span style="color: #0000BB">who</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 /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">__CLASS__</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #0000BB">B</span><span style="color: #007700">::</span><span style="color: #0000BB">test</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
A
</pre></div>
</div>
</div>
</div>
<div class="sect2" id="language.oop5.late-static-bindings.usage">
<h3 class="title">Late Static Bindings' usage</h3>
<p class="para">
Late static bindings tries to solve that limitation by introducing a
keyword that references the class that was initially called at runtime.
Basically, a keyword that would allow you to reference
<em>B</em> from <em>test()</em> in the previous
example. It was decided not to introduce a new keyword but rather use
<em>static</em> that was already reserved.
</p>
<div class="example" id="example-229">
<p><strong>Example #2 <em>static::</em> simple usage</strong></p>
<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 /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">__CLASS__</span><span style="color: #007700">;<br /> }<br /> public static function </span><span style="color: #0000BB">test</span><span style="color: #007700">() {<br /> static::</span><span style="color: #0000BB">who</span><span style="color: #007700">(); </span><span style="color: #FF8000">// Here comes Late Static Bindings<br /> </span><span style="color: #007700">}<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 /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">__CLASS__</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #0000BB">B</span><span style="color: #007700">::</span><span style="color: #0000BB">test</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
B
</pre></div>
</div>
</div>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
In non-static contexts, the called class will be the class of the object
instance. Since <em>$this-></em> will try to call private
methods from the same scope, using <em>static::</em> may give
different results. Another difference is that <em>static::</em>
can only refer to static properties.
</p>
</p></blockquote>
<div class="example" id="example-230">
<p><strong>Example #3 <em>static::</em> usage in a non-static context</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">A </span><span style="color: #007700">{<br /> private function </span><span style="color: #0000BB">foo</span><span style="color: #007700">() {<br /> echo </span><span style="color: #DD0000">"success!\n"</span><span style="color: #007700">;<br /> }<br /> public function </span><span style="color: #0000BB">test</span><span style="color: #007700">() {<br /> </span><span style="color: #0000BB">$this</span><span style="color: #007700">-></span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /> static::</span><span style="color: #0000BB">foo</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 /> </span><span style="color: #FF8000">/* foo() will be copied to B, hence its scope will still be A and<br /> * the call be successful */<br /></span><span style="color: #007700">}<br /><br />class </span><span style="color: #0000BB">C </span><span style="color: #007700">extends </span><span style="color: #0000BB">A </span><span style="color: #007700">{<br /> private function </span><span style="color: #0000BB">foo</span><span style="color: #007700">() {<br /> </span><span style="color: #FF8000">/* original method is replaced; the scope of the new one is C */<br /> </span><span style="color: #007700">}<br />}<br /><br /></span><span style="color: #0000BB">$b </span><span style="color: #007700">= new </span><span style="color: #0000BB">B</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$b</span><span style="color: #007700">-></span><span style="color: #0000BB">test</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$c </span><span style="color: #007700">= new </span><span style="color: #0000BB">C</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$c</span><span style="color: #007700">-></span><span style="color: #0000BB">test</span><span style="color: #007700">(); </span><span style="color: #FF8000">//fails<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
success!
success!
success!
Fatal error: Call to private method C::foo() from context 'A' in /tmp/test.php on line 9
</pre></div>
</div>
</div>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Late static bindings' resolution will stop at a fully resolved static call
with no fallback. On the other hand, static calls using keywords like
<em>parent::</em> or <em>self::</em> will forward the
calling information.
</p>
<div class="example" id="example-231">
<p><strong>Example #4 Forwarding and non-forwarding calls</strong></p>
<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 /> public static function </span><span style="color: #0000BB">foo</span><span style="color: #007700">() {<br /> static::</span><span style="color: #0000BB">who</span><span style="color: #007700">();<br /> }<br /><br /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">__CLASS__</span><span style="color: #007700">.</span><span style="color: #DD0000">"\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 /> public static function </span><span style="color: #0000BB">test</span><span style="color: #007700">() {<br /> </span><span style="color: #0000BB">A</span><span style="color: #007700">::</span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /> </span><span style="color: #0000BB">parent</span><span style="color: #007700">::</span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /> </span><span style="color: #0000BB">self</span><span style="color: #007700">::</span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /> }<br /><br /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">__CLASS__</span><span style="color: #007700">.</span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /> }<br />}<br />class </span><span style="color: #0000BB">C </span><span style="color: #007700">extends </span><span style="color: #0000BB">B </span><span style="color: #007700">{<br /> public static function </span><span style="color: #0000BB">who</span><span style="color: #007700">() {<br /> echo </span><span style="color: #0000BB">__CLASS__</span><span style="color: #007700">.</span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #0000BB">C</span><span style="color: #007700">::</span><span style="color: #0000BB">test</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
A
C
C
</pre></div>
</div>
</div>
</p></blockquote>
</div>
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