android ad function customiation

We already have a app made in eclipse. What we want is: Ad function changes: if a given(already coded) google ad code does not respond with ad or that add unit id is not working, this app function should go to predefined web URL (php server, i will give a shared hosting server you can add yr scripts or whatever you want der, but i should get a field to add new ad unit id) and fetch a new ad unit id code from that and ad delivery starts from that NEW ad unit id. This should update code in app’s local database in phone so once this function updated the code, from next time onwards, every time it should load ads from new ad unit id or code. So code is fetched only once from the server., next time when this code stops working, then new code (ad unit id) is fetched. We have 2 ad units banner and interstitial.