Function? How to find if xy values are within a coordinates of polygon?

I'm making a map (image) which you can click on and the script finds the area-code based on where you click on the map. Before I spend time on writing my own code to find if xy values are within a certain coordinates of a polygon, do you know of any build in function to do it? I have browsed lightly through the PHP-manual but not found any. Thanks, SED