To add Google Analytics to PHPBB, edit
styles/$theme/template/overall_footer.html
<script src="http://www.google-analytics.com/ga.js" type="text/javascript"></script><script type="text/javascript">try{var g=_gat._getTracker('$analytics-id');g._trackPageview();}catch(ex){}</script>
</body>
Purge the cache in the ACP | General tab