Limit PHP requests per IP

Hello

We seen occasional crawlers, possible searching for vulnerabilities, requesting many PHP pages at once on different domains.
This causes load to spike, and eventually long (>10 seconds) load times for legitimate requests.

I was wondering if there is an apache module that could limit PHP process requests per remote IP.
for example: 4 PHP requests at once / IP, consecutive requests should be queued for later processing.
And excluding other requests (like CSS, images, html,...)

This should be a sort of QOS-like system to prevent overloading.

Thank you in advance.

Kind regards
Dries