It's hard to tell from Apache general access_log
which requests are processed by PHP, because direct file hits muddy the waters.
PHP FPM SAPI has a super useful access_log
directive, but how do I get the same effect using PHP CGI SAPI?
PHP and Apache level solutions are both fine.
You haven't even hinted your OS, I assume you're on UNIX-type system.
One easy to implement and universal way is to grep your general access log regularily with ie. cron. If you extend your question and provide sample log lines I could help with finding a correct regexp and command. Things to take into account here are logrotate (avoid loosing some logs on rotate) and a need to grep with low priority (avoid generating spikes of high load).
I use the above method to provide log snippets for our web developers and it haven't fallen short yet.