The ad script outputs text before PHPBB does. PHPBB assumes that nothing has outputted text yet, so it outputs headers, which are special commands to the client. Headers do not work if they are not sent as the first text on the page. PHPBB compressed the pages it sends to the client, so it has to tell the client to decompress the pages with a header. However, since the headers are the not the first text sent, they break, and the client doesn't know it has to decompress the data it recieves, so it tries to output it as plain text, which is what you see.