I'm currently getting the error "Your map [maps/ph_zombie_bunker.bsp] differs from the server's." I honestly have no idea what to do.

Oh boy, when an updated map replaces the old, let's just use the same exact name, not use a name that won't cause this fuggin' conflict. Lessee what I can do. Head to: C://Windows/Program Files(x86)/Steam/steamapps/common/GarrysMod/garrysmod/download/maps.  Find that map in question and delete it. Rejoin the server at that point to receive the updated map. Hope this works!

Another thing you can do is find the maps in question or the whole PH server and subscribe to them all in the workshop and that fixes the problem as well sometimes It's what I did and much simpler since the workshop will replace the file with the updated.