Source of: /manual/en/language.oop5.static.php
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<div id="language.oop5.static" class="sect1">
<h2 class="title">Static Keyword</h2>
<p class="para">
Declaring class properties or methods as static makes them accessible
without needing an instantiation of the class. A property declared as
static can not be accessed with an instantiated class object (though
a static method can).
</p>
<p class="para">
For compatibility with PHP 4, if no <a href="language.oop5.visibility.php" class="link">visibility</a>
declaration is used, then the property or method will be treated
as if it was declared as <i>public</i>.
</p>
<p class="para">
Because static methods are callable without an instance of
the object created, the pseudo-variable <var class="varname">$this</var> is
not available inside the method declared as static.
</p>
<p class="para">
Static properties cannot be accessed through the object using the arrow
operator ->.
</p>
<p class="para">
Calling non-static methods statically generates an E_STRICT level warning.
</p>
<p class="para">
Like any other PHP static variable, static properties may only be
initialized using a literal or constant; expressions are not
allowed. So while you may initialize a static property to an
integer or array (for instance), you may not initialize it to
another variable, to a function return value, or to an object.
</p>
<p class="para">
As of PHP 5.3.0, it's possible to reference the class using a variable.
The variable's value can not be a keyword (e.g. <i>self</i>,
<i>parent</i> and <i>static</i>).
</p>
<div class="example">
<p><b>Example #1 Static property example</b></p>
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">Foo<br /></span><span style="color: #007700">{<br /> public static </span><span style="color: #0000BB">$my_static </span><span style="color: #007700">= </span><span style="color: #DD0000">'foo'</span><span style="color: #007700">;<br /><br /> public function </span><span style="color: #0000BB">staticValue</span><span style="color: #007700">() {<br /> return </span><span style="color: #0000BB">self</span><span style="color: #007700">::</span><span style="color: #0000BB">$my_static</span><span style="color: #007700">;<br /> }<br />}<br /><br />class </span><span style="color: #0000BB">Bar </span><span style="color: #007700">extends </span><span style="color: #0000BB">Foo<br /></span><span style="color: #007700">{<br /> public function </span><span style="color: #0000BB">fooStatic</span><span style="color: #007700">() {<br /> return </span><span style="color: #0000BB">parent</span><span style="color: #007700">::</span><span style="color: #0000BB">$my_static</span><span style="color: #007700">;<br /> }<br />}<br /><br /><br />print </span><span style="color: #0000BB">Foo</span><span style="color: #007700">::</span><span style="color: #0000BB">$my_static </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$foo </span><span style="color: #007700">= new </span><span style="color: #0000BB">Foo</span><span style="color: #007700">();<br />print </span><span style="color: #0000BB">$foo</span><span style="color: #007700">-></span><span style="color: #0000BB">staticValue</span><span style="color: #007700">() . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br />print </span><span style="color: #0000BB">$foo</span><span style="color: #007700">-></span><span style="color: #0000BB">my_static </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">; </span><span style="color: #FF8000">// Undefined "Property" my_static <br /><br /></span><span style="color: #007700">print </span><span style="color: #0000BB">$foo</span><span style="color: #007700">::</span><span style="color: #0000BB">$my_static </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$classname </span><span style="color: #007700">= </span><span style="color: #DD0000">'Foo'</span><span style="color: #007700">;<br />print </span><span style="color: #0000BB">$classname</span><span style="color: #007700">::</span><span style="color: #0000BB">$my_static </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">; </span><span style="color: #FF8000">// As of PHP 5.3.0<br /><br /></span><span style="color: #007700">print </span><span style="color: #0000BB">Bar</span><span style="color: #007700">::</span><span style="color: #0000BB">$my_static </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$bar </span><span style="color: #007700">= new </span><span style="color: #0000BB">Bar</span><span style="color: #007700">();<br />print </span><span style="color: #0000BB">$bar</span><span style="color: #007700">-></span><span style="color: #0000BB">fooStatic</span><span style="color: #007700">() . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<div class="example">
<p><b>Example #2 Static method example</b></p>
<div class="example-contents programlisting">
<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">Foo </span><span style="color: #007700">{<br /> public static function </span><span style="color: #0000BB">aStaticMethod</span><span style="color: #007700">() {<br /> </span><span style="color: #FF8000">// ...<br /> </span><span style="color: #007700">}<br />}<br /><br /></span><span style="color: #0000BB">Foo</span><span style="color: #007700">::</span><span style="color: #0000BB">aStaticMethod</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$classname </span><span style="color: #007700">= </span><span style="color: #DD0000">'Foo'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$classname</span><span style="color: #007700">::</span><span style="color: #0000BB">aStaticMethod</span><span style="color: #007700">(); </span><span style="color: #FF8000">// As of PHP 5.3.0<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
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