migrating from register_globals on to off

Hello, Lately we discovered that, by mistake, we've run PHP with register_globals on for a few months for all our customers. Since we want to turn it off, we need to figure out which customers use the 'register_globals' functionality so we can ask them to modify their scripts. However, checking all php code by hand is a non-trivial task, because there are 20,000+ php scripts. Is there a way to: a) parse all .php scripts once and detect whether they depend on register globals on? or probably easier: b) add something to our current setup so that when a scripts is executed and uses the register globals functionality, we get a notification automatically? Any suggestions? Thanks!