If your starting httpd from the redhat init.d scripts, just change the script so it makes the changes for you.
Make 2 httpd.conf files, httpd.conf-ORIG and httpd.conf-PHP. ORIG is a non php enabled conf file, and PHP has the php modules added.
The startup script in /etc/rc.d/init.d is just a bash script, so you can have it copy the ORIG file to httpd.conf and then start httpd, then tell it to sleep for a few seconds to allow httpd to start up. After it sleeps a few seconds copy the PHP file to httpd.conf and run killall -HUP httpd and you'll have apache running with php support.
I had to do this a while back on my system. I could not figure out what was killing httpd at startup, but this hack got the job done. I'd add the script code to this message but I don't have it handy. If you need a hand doing the above drop me a line and I'll come up with something.
Good luck.