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Why is this an issue?
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();  // Prints "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();  // Prints "Balls are fun!"

Exceptions
No issue is raised when self is used on a constant field, a private field or a private method.
class A
{
    private static $somevar = "hello";
    const CONSTANT = 42;

    private static function foo()
    {
        $var = self::$somevar . self::CONSTANT;  // Should be OK
        self::foo();                               // Should be OK
    }
}