When a file is closed the lock will be released by the system anyway, even if PHP doesn't do it explicitly anymore since 5.3.2 (the documentation is very confusing about this).
However, I had a situation on an apache/PHP server where an out-of-memory error in PHP caused file handles to not be closed and therefore the locks where kept even thought PHP execution had ended and the process had returned to apache to serve other requests. The lock was kept alive until apache recycled those processes.
This lack of proper clean up basically makes flock() completely unreliable.
