Memory Leakage in PHP - Garbage Collector
A memory leak is a specific type of problem where something is put in memory but never removed. This is much less a problem for a HTTP request as when the request is complete the process goes away so even if you have a memory leak it likely wont matter much. They are much more problematic for a long running process.
PHP is a pretty good at garbage collection so memory leaks aren't too common but they do happen from time to time.
Generally where you need to watch out for them is in loops. To avoid memory leaks in your scripts you really just need to close any open file streams, destroy image resources, or close any open socket connections when you no longer need them. Also if you have a variable that takes up a large amount of memory you might want to unset it yourself one you don't need it anymore rather than wait for the garbage collector to do it for you.
Overall this isn't something that need too be worries about. Thanks!