self:: vs className:: inside static className methods in PHP

I guess there may not be any difference but personal preference, but when reading various PHP code I come across both ways to access the methods class.

What is the difference:

class Myclass
{
    public static $foo;

    public static function myMethod ()
    {
        // between:
        self::$foo;
        // and
        MyClass::$foo;
    }
}