I was getting a new server up and running and wanted to install APC (Alternative PHP Cache). Things were going great. I even ran phpinfo() and noted that APC had an entry. Everything was super, right?
I wanted more confirmation, so I used SSH, located apc.php and copied it to a domain for public access. I wanted to know how well APC was working for me. However, it wouldn't run. It sent back errors and when I tried to debug it I got no where.
Turns out, APC wasn't really doing much of anything. I had PHP installed and running as a CGI instead of an Apache Module. If PHP runs as a CGI, APC closes at the end of each script run and the cache is lost—it doesn't make much sense to use APC in this situation. While runing as a CGI has some security benefits you lose a bit of performance and, more importantly to me, APC doesn't really work.
I use WHM and so I went to "Main >> Service Configuration >> Apache Configuration" and then changed the PHP 5 handler to DSO. This solved the problem and apc.php loaded just fine the next time I tried. And it even had some pretty stats for me to see.