static::staticFunctionName()

I know what self::staticFunctionName() a开发者_JS百科nd parent::staticFunctionName() are, and how they are different from each other and from $this->functionName.

But what is static::staticFunctionName()?

It's the keyword used in PHP 5.3+ to invoke late static bindings.
Read all about it in the manual: http://php.net/manual/en/language.oop5.late-static-bindings.php

In summary, static::foo() works like a dynamic self::foo().

class A {
    static function foo() {
        // This will be executed.
    }
    static function bar() {
        self::foo();
    }
}

class B extends A {
    static function foo() {
        // This will not be executed.
        // The above self::foo() refers to A::foo().
    }
}

B::bar();

static solves this problem:

class A {
    static function foo() {
        // This is overridden in the child class.
    }
    static function bar() {
        static::foo();
    }
}

class B extends A {
    static function foo() {
        // This will be executed.
        // static::foo() is bound late.
    }
}

B::bar();

static as a keyword for this behavior is kind of confusing, since it's all but. :)