Targeting audience php MySQL


I’m a newbie in php and MySQL. I’m trying to implement a osm feature in my script.
I have a small video website and I want the users to filter their audience, so only other users from a selected place could watch the video, using a radius circle around the node (the user chooses the node place). A very nice example of that is the Facebook’s advertising system, where you filter the audience for your ad, and while you are moving the circle range that shows the number of people inside that range. I don’t know exactly the name of the feature I’m looking for.