...but being exact, the return value of strstr() isn't a boolean true either - it's only casted to true in the if-statement.
Apparently Jeewhizz confused strstr() with strpos(), because with strpos() you have to be wary of a returned 0 which gets casted to false, that's why you should use !== instead of !=.
http://www.php.net/manual/en/function.strpos.php