You must include a copy of the licence

You must leave all existing copyrights in the source in place and unmodified.

You may not directly append your copyright to ours or try, in any way, shape or form to suggest you "co-wrote" the original source ... you may add your own copyright notice on a seperate line.

You must leave all existing references to the licence intact and unaltered.

You may not relicence any part of phpBB without our express permission (don't bother asking, we won't allow it )

You must clearly note that you have changed the source code. This does not mean dropping a timestamp ID into our header comments. It means a highly visible comment noting that you altered this code and when. The more typical approach is to also include an overview of the changes.

You must include copyright details of source code you have incorporated from third parties, e.g. Mods, images, etc. Remember that not all images, source is released under the GPL and therefore you may be in breach of someone elses licence/copyright if you haven't first obtained permission to use them

You may not force those obtaining your modified work to abide by any additional conditions, i.e. you cannot prevent people redistributing your modified work for free under the GPL.