php output to string

Hi there, i'm a little newbie you know. I have a php that returns some xml, and i want to parse that xml through php. the xml returned depends on the variable passed thru the url. what i need is to get the output of that php file and store it in a string. i have tried everything i have though, readfile, include, file_get_contents even eval. also i have tried with ob_start and so. but no luck. how can i do this? thanks