The PHP flock documentation says it is unsafe on some multithreaded servers like ISAPI, because the lock is a process wide thing, so if one instance of php on thread 1 locks a file another php instance on thread 2 may also lock it because from the OS's point of view the same process already owns the lock.
Now, the PHP docs say this is only a problem on some operating systems, and names ISAPI as an example. My question: can this also happen on linux/apache in multithreaded mode, or is flock safe in that case?