I found a Google Adsense extension for phpBB.
How to add Google Adsense to phpBB
About the Author: Matt Woicik
Matt is a digital marketing strategist and the owner of ML2 Solutions, a digital marketing agency. He helps businesses increase awareness and get more leads using digital marketing. New marketing methods are confusing, always changing and difficult to figure out. Matt helps to explain things and provide solutions with clear results.