Beginning May 1, the AirTrain monorail to Newark Liberty Airport will be closed for repairs. How can I get from Philadelphia to Newark Airport in this situation?
As of Thursday July 3, 2014 the EWR AirTrain is back in operation.
The Port Authority of NY/NJ press release on the shutdown includes the high-level details of the shutdown and the alternatives - including shuttle buses that will be added, along with standard bus lines that are available.
At this stage they have not announced many details of the shuttles that will be run, however I would expect they will in the next week or so. They will almost certainly have people available to direct/assist passengers, however I would expect the trip to take longer than normal, especially during peak traffic hours.