date_default_timezone_set thread safe?

My application stores all dates/times in UTC. And every user has a timezone setting (which can be different than the local timezone on his machine, ie the user will see the same time regardless from where and what machine he logs in).

Now if a user requests data with a timestamp, I use date_default_timezone_set to be able to convert from UTC to his timezone.

I am wondering how this setting is stored. More specifically, could there be thread safety issue (if a webserver is running multiple php threads)?

(Note: I have no reason to suspect the function is not thread safe - I just want to make sure as I don’t see it documented and I am not an expert in how PHP stores “global” variables)