He asks the question: "Is there significant overhead to the is_ functions like is_array, is_object, is_string in php?" He wonders if those kinds of functions, since they are on such a base level, could cause more trouble than their worth when used en masse. He asks because it's used heavily in an approach for his developing Ookles application.

There's already been one comment made on the post, and it mentions that they are, in fact, a trivial call to PHP. It's a simple test that validates simply without much need for too much background action. It does mentions some caveats - is_object, is_resource, and, at times, is_callable.