I'll agree with cab747.. the connection in itself is EXTREMELY tight, there is an interterminal bus as mentioned, but between wait times for the bus, and in the case your inbound LAX flight is a little delayed you will be in trouble. The saving grace is, that time of night there shouldn't be any delay with security, but make sure you get your connecting BP - the US ticket counters are known to be packed with pax and they don't have much agents serving them as it's just a late departure. Then again, at that time most if not all pax should be checked in.

Bottom line: get an earlier flight if you can - the distance itself between the commuter terminal and T2 (and checkin is at the very, very end of T2), as well as other logistical stuff you may run out of time. Being that your PHL flight is the last one of the day, there is no margin for error.