To correct manolachef's answer: define() ALWAYS defines constants in the GLOBAL namespace.
As nl-x at bita dot nl states in the note at http://www.php.net/manual/en/function.define.php, the constant "NULL" can be defined with define() case-sensitively, but can only be retrieved with constant(), leaving the meaning of NULL uppercase keyword as the only value of the type null.