In case you don't understand why you need to "Avoid using this function in multithreaded webservers":

It's because this function changes the umask at the process level, rather than only for PHP or for the current script.  If there are multiple simultaneous threads running in the process in which your PHP script is running, the change will apply to all of those threads at the same time hence why this is not safe for multithreaded use.

I understand that if you are using the PHP module and Apache's prefork MPM, which is not multi-threaded, then you at least won't get race-condition problems such as this.  However, it is still worth noting that the umask setting, if not re-set, will persist for the life of that process even if the process is re-used to serve future PHP or non-PHP requests.