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.
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:
A
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:
B
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, usingstatic::may give different results. Another difference is thatstatic::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();
$c->test(); //fails
?>The above example will output:
success! success! success! Fatal error: Call to private method C::foo() from context 'A' in /tmp/test.php on line 9
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::orself::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