With the upgrade to php 7 comes the deprecation of the old mongodb driver. The new driver has existed in parallel with it, but nobody bothered to switch their stuff.
The new driver is pretty low level and not that user friendly, so they also released a library that sits on top of it that replaces some of the usability features of the old driver. Which is nice of them. Unfortunately this stuff isn't backwards compatible.
There is a library that I was using in development that works on top of the old driver. It does not work on the new driver, not even on top of the new library. I can't even get it to work, the differences in the new driver are too extreme.
I'm left with the choices to wait for him to get around to updating the library to use the new driver, try to go it on my own without that library (which was REALLY perfect), or roll back to php *wrinkles nose* 5.6.
I don't like any of them. :(