Emulating the db.currentOp() shell method on older MongoDB versions (without the currentOp command) requires a query to the admin database's $cmd.sys.inprog collection. I encountered the following assertion failure when attempting to query the current op from the PHPC driver:

AssertionException handling request, closing client connection: 16810 bad query: BadValue unknown top level operator: $query

From the driver's perspective, the socket is closed and we time out while attempting to read a response.

The assertion is no doubt due to logic in the PHPC driver that always includes the filter under a top-level $query field, even if the query contains no other modifiers necessitating so (e.g. $orderby, modifiers). While this is certainly something we can fix in PHPC, I wonder if the the server should still relax its assertion here.

I've linked this to SERVER-17951, where the server was rejecting similar queries due to a top-level $readPreference field. If this is deemed worthy of a fix, you may want to also investigate the killOp and fsyncUnlock operations, which were also addressed in SERVER-17951.