Static methods/variables and class constants are bound to the class therefore we must use self to reference the current class, $this however references the current object not class, remember there is a fundamental difference between the two; that is why you can't use $this to access methods/properties marked as static or class constants because they are bound to the class not the object.

__________________
mysql> SELECT * FROM `users` WHERE `users`.`clue` > 0;
Empty set (0.00 sec)