Files Not Being Included With Set_include_path() Or Ini_set('include_path')

I cannot seem to wrap my head around this peculiar problem. I have tried using both the function to set my include path, and then echo the get_included_files() and the files do not appear in the array. I can use require() or include() with no problems, but when I try to set the include path, the files do not seem to be included. Is there some weird quirck or gotcha to using this feature of PHP? I thought all you have to do is provide the directory and viola it should work. Any advice?