It depends on how you installed Apache and PHP.
Generally:
Yes, it's certainly possible to have a modular Apache and PHP installation.
The Apache in Red Hat Linux (and all other Linux-distributions, I suppose) is very modular, and you may add/remove modules by simply editing Apache's httpd.conf file and restarting Apache.
In the same way, PHP may be compiled in a modular way. See for example http://rpms.arvin.dk/php/