I need to install a joomla extension that requires php5.
Tell us, which version of the joomla CMS are you using (1.0.15/1.5.21/1.6 beta)? And which joomla extension and version did you install? If the use of PHP5 wrecked your joomla menu then it should already have been wrecked immediately
after installing/enabling thte PHP5 contrib and
before you installed the joomla extension. Did you verify/notice this? Are you sure it is not the joomla extension which caused this problem?
Try troubleshooting as follows: temporarily uninstall the new joomla extension and verify if the menu problem persists. If the menu works again the joomla extension is the culprit. If the menu still doesn't work, disable the PHP5 functionality (uninstalling the PHP5 contrib is not necessary) for the joomla CMS and see if the menu is still not functioning properly.
Also check the messages log (in /var/log) and the error log (in /var/log/httpd) for PHP related error messages.
Does anyone know if php5 is the new default for SME 8? If so, maybe I could use this opportunity to upgrade SME and joomla.
PHP5 is the new default for SME8. SME8 is still in beta however.