References in a class to static class members (fields or methods) can be made using either self::$var or static::$var (introduced in 5.3). The difference between the two is one of scope. Confusingly, in subclasses, the use of self:: references the original definition of the member, i.e. the superclass version, rather than any override at the subclass level. static::, on the other hand, references the class that was called at runtime.

Noncompliant Code Example

<?php

class Toy {

    public static function status() {
        self::getStatus();                  // Noncompliant; will always print "Sticks are fun!" even when called from a subclass which overrides this method;
    }

    protected static function getStatus() {
        echo "Sticks are fun!";
    }
}

class Ball extends Toy {

    protected static function getStatus() { // Doesn't actually get called
        echo "Balls are fun!";
    }
}

$myBall = new Ball();
$myBall::status();                          // Will print "Sticks are fun!"

Compliant Solution

<?php

class Toy {

    public static function status() {
        static::getStatus();   // Compliant
    }

    protected static function getStatus() {
        echo "Sticks are fun!";
    }
}

class Ball extends Toy {

    protected static function getStatus() {
        echo "Balls are fun!";
    }
}

$myBall = new Ball();
$myBall::status();             // Will print "Balls are fun!"

Exceptions

No issue is raised when self is used in a class field declaration

class A {
   const CONSTANT;

   public $arr = array(
        self::CONSTANT         // Compliant
   );
}