First of all ensure that the RPMS are installed:
rpm -qa|grep php mod_php3-3.0.9-1 mod_php3-imap-3.0.9-1
Then you will need to go into your /etc/httpd/conf/httpd.conf and uncomment the appropriate lines to enable them. A full installation procedure is in your manual. For information on configuring the IMAP RPM that's listed above see the Web mail link from the main FAQ page.