I need to add MySQL support to my PHP installation on RHEL4. I need to rebuild PHP from source due to other extensions that are required. Will Apache be down for a few seconds when I restart it, or will I need to stop it while I install PHP?
It depends on whether you are using mod_php or php cgi. Mod_php is loaded into memory when apache starts, so you can install php without affecting it. The new mod_php module will be loaded when you restart apache.
PHP CGI on the other hand gets loaded every time a script runs. If you are installing PHP to a new location, that's fine, because apache will only look for the new location when it is restarted and finds the new location in its config file. However, if you are using PHP CGI and installing php over an existing installation, you will have to bring down apache while that happens.