php-doc-en/language/oop5/static.xml

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 <sect1 id="language.oop5.static">
  <title>Static Keyword</title>

  <para>
   Declaring class members or methods as static, makes them callable
   from outside the object context. A member or method declared
   with static can not be accessed with a variable that is an instance
   of the object and cannot be re-defined in an extending class.
  </para>

  <para>
   The static declaration must be after the visibility declaration. For 
   compatibility with PHP 4, if no <link
   linkend="language.oop5.visibility">visibility</link>
   declaration is used, then the member or method will be treated
   as if it was declared as <literal>public static</literal>.
  </para>

  <para>
   Because static methods are callable without an instance of
   the object created, the pseudo variable <varname>$this</varname> is
   not available inside the method declared as static.
  </para>
  
  <para>
   In fact <literal>static</literal> method calls are resolved at compile 
   time. When using an explicit class name the method is already identified
   completley and no inheritance rules apply. If the call is done by 
   <literal>self</literal> then <literal>self</literal> is translated to 
   the current class, that is the class the code belongs to. Here also no 
   inheritance rules apply.
  </para>
  
  <para>
   Static properties cannot be accessed through the object using the arrow
   operator -&gt;.
  </para>

  <example>
   <title>Static member example</title>
   <programlisting role="php">
<![CDATA[
<?php
class Foo
{
    public static $my_static = 'foo';

    public function staticValue() {
        return self::$my_static;
    }
}

class Bar extends Foo
{
    public function fooStatic() {
        return parent::$my_static;
    }
}


print Foo::$my_static . "\n";

$foo = new Foo();
print $foo->staticValue() . "\n";
print $foo->my_static . "\n";      // Undefined "Property" my_static 

// $foo::my_static is not possible

print Bar::$my_static . "\n";
$bar = new Bar();
print $bar->fooStatic() . "\n";
?>
]]>
   </programlisting>
  </example>

  <example>
   <title>Static method example</title>
    <programlisting role="php">
<![CDATA[
<?php
class Foo {
    public static function aStaticMethod() {
        // ...
    }
}

Foo::aStaticMethod();
?> 
]]>
    </programlisting>
  </example>

 </sect1>
 
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