According to the manual page of flock() that PHP uses internally, a lock is released when either flock() is called with LOCK_UN or when the descriptor is closed using fclose().
Upon script termination, either PHP or the OS will close the open file descriptors, thereby releasing the locks you may have.
Because of said behaviour this commit (5.3) and this one (5.2) were made to no longer do the unlocking in PHP itself.