External.php modification

[Electronic Punk]

I am trying to create an option in external.php so that I can only display threads with a specific prefixid.

I made a copy of external.php and renamed it and started work, but it seems it is both creating and using cached data so it affected the standard external.php which I really don't want to happen.

Does anyone have any ideas or know of an existing modification? Been trying to do this for a few hours and am both running out of ideas and getting out of my php comfort zone