Querying an object's functions

Is there any way in PHP 4.0.6 to query an object for its member functions? For example I have a couple classes that have an htmlString() function to display themselves in some special way as html. I'd like to go through a list of various objects, find out if each can run htmlString() and if so use that, otherwise use some generic object-to-html function. I could just put a member variable in the object and check for that, but it would be more elegant to be able to check for the function directly. Thanks for any suggestions. -Steve