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Late Static Bindings

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.

This feature was named "late static bindings" with an internal perspective in mind. "Late binding" comes from the fact that static:: will no longer 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.

Limitations of self::

Static references to the current class like self:: or __CLASS__ are resolved using the class in which the function belongs, as in where it was defined:

Example #1 self:: usage

 <?php
class A {
    public static function who() {
        echo __CLASS__;
    }
    public static function test() {
        self::who();      
    }  
}  

class B extends A {      
    public static function who() {
         echo __CLASS__;
    }  
}   

B::test();
?>

The above example will output:

Late Static Bindings' usage

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 B from test() in the previous example. It was decided not to introduce a new keyword but rather use static that was already reserved.

Example #2 static:: simple usage

 <?php
class A {
    public static function who() {
        echo __CLASS__;
    }
    public static function test() {
        static::who(); // Here comes Late Static Bindings     
    }  
}  

class B extends A {      
    public static function who() {
         echo __CLASS__;
    }  
}   

B::test();
?>

The above example will output:

Note: static:: does not work like $this for static methods! $this-> follows the rules of inheritance while static:: doesn't. This difference is detailed later on this manual page.

Example #3 static:: usage in a non-static context

 <?php
class TestChild extends TestParent {
    public function __construct() {
        static::who();
    }

    public function test() {
        $o = new TestParent();
    }

    public static function who() {
        echo __CLASS__."\n";
    }
}

class TestParent {
    public function __construct() {
        static::who();
    }

    public static function who() {
        echo __CLASS__."\n";
    }
}
$o = new TestChild;
$o->test();

?>

The above example will output:

  TestChild TestParent

Note: Late static bindings' resolution will stop at a fully resolved static call with no fallback.
Example #4 Fully resolved static calls

<?php class A { public static function foo() { static::who(); } public static function who() { echo __CLASS__."\n"; } } class B extends A { public static function test() { A::foo(); } public static function who() { echo __CLASS__."\n"; } } B::test(); ?>

The above example will output:
  A 
      

Edge cases

There are lots of different ways to trigger a method call in PHP, like callbacks or magic methods. As late static bindings base their resolution on runtime information, it might give unexpected results in so-called edge cases.

Example #5 Late static bindings inside magic methods

 <?php
class A {

   protected static function who() {
        echo __CLASS__."\n";
   }

   public function __get($var) {
       return static::who();
   }
}

class B extends A {

   protected static function who() {
        echo __CLASS__."\n";
   }
}

$b = new B;
$b->foo;
?>

The above example will output: