Most web workloads will use some sort of process cache, like php-fpm, simply because spawning a process for each request is very inefficient. So the process and any extensions used can leak memory over time across requests.
php-fpm is typically configured to restart those processes after some number of requests, just in case there is such a leak. But I have not seen those leaks happen between process restarts, state is still wiped clean between requests, so leaks should be rare/difficult and probably most likely the result of faulty extensions.