(Since this function wasn't documented as of the date I left this note...)
If you came here looking for documentation, allow me to point you instead to a section in the user manual about garbage collection that includes a bit at the end about when you would use this feature and what it will actually do: http://php.net/manual/en/features.gc.collecting-cycles.php
Hope it helps!