I had been reading about Apache graceful restart.
apachectl -k graceful
The USR1 or graceful signal causes the parent process to advise the children to exit after their current request (or to exit immediately if they're not serving anything).
...meaning that anyone who is in the middle of downloading something will continue to be able to download it.
If a long running PHP script is currently being executed (eg: processing a large image), will graceful restart allow it to be completed or will it hinder its execution?
It will be completed because the Apache child process is busy serving the response for the customers request and waits for PHP to finish the response.