Late Static Bindings

PHP implements a feature called late static bindings which can be used to reference the called class in a context of static inheritance.

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 :: 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 self::, parent::, static::, or, if going up in the class hierarchy, forward_static_call(). The function get_called_class() can be used to retrieve a string with the name of the called class and static:: introduces its scope.

This feature was named "late static bindings" with an internal perspective in mind. "Late binding" comes from the fact that static:: 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.

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 referencing 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:
In non-static contexts, the called class will be the class of the object instance. Since $this-> will try to call private methods from the same scope, using static:: may give different results. Another difference is that static:: can only refer to static properties.

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

<?php

class A
{
    private function foo()
    {
        echo "Success!\n";
    }

    public function test()
    {
        $this->foo();
        static::foo();
    }
}

class B extends A
{
    /* foo() will be copied to B, hence its scope will still be A and
    * the call be successful */
}

class C extends A
{
    private function foo()
    {
        /* Original method is replaced; the scope of the new one is C */
    }
}

$b = new B();
$b->test();

$c = new C();
try {
    $c->test();
} catch (Error $e) {
    echo $e->getMessage();
}

?>

The above example will output:

Success!
Success!
Success!
Call to private method C::foo() from scope A

Note:
Late static bindings' resolution will stop at a fully resolved static call with no fallback. On the other hand, static calls using keywords like parent:: or self:: will forward the calling information.
Example #4 Forwarding and non-forwarding 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(); parent::foo(); self::foo(); } public static function who() { echo __CLASS__ . "\n"; } } class C extends B { public static function who() { echo __CLASS__ . "\n"; } } C::test(); ?>

The above example will output:
A
C
C

Late Static Bindings

Limitations of self::

Late Static Bindings' usage

Limitations of `self::`