En realidad en PHP recomiendan usar:
if (null === $var)
http://hakre.wordpress.com/2011/03/2..._null-vs-null/ http://www.php.net/manual/es/function.is-null.php
Using === NULL instead of is_null(), is actually useful in loaded server scenarios where you have hundreds or thousands of requests per second. Saving microseconds on a lot of "simple" operations in the entire PHP execution chain usually results in being able to serve more pages per second at the same speed, or lowering your cpu usage. People usually write very bad and slow code.
Saludos