PHL departure controller experienced a near conflict between departure/arrival air carriers when the tower apparently issued an incorrect initial heading with the take off clearance; reporter advocating a return to a SID.

Narrative

I was working South Departure combined with South High and DQO. PHL was in a converging Runway 9R/17 configuration. Air Carrier X; a heavy checked in climbing to 5;000. I RADAR identified him and gave him a climb to 10;000 with a right turn direct Ditch leaving 3;000. Air Carrier X questioned this right turn which made me take a closer look. It appeared that Air Carrier X was in a turn to the southwest right into Aircraft Y; a PHL arrival at 4000. I immediately told Air Carrier X to turn left direct DITCH and stop his climb at 3000. He responded with we are already through 3000 and in a turn direct DITCH. It appeared that I had the required 5 miles wake turbulence separation so I had Air Carrier X stop at 5000 to avoid additional arrivals at 6000. I also told Air Carrier X to fly heading 090. When I questioned Air Carrier X about his initial heading; he said the Tower had given him a right turn heading 245. Air Carrier X also said he had questioned the Tower about that heading since they were departing on Runway 9 and the Tower again issued a right turn to 245. Recommendation; when aircraft call Clearance Delivery; they are given the PHL8 Departure. Then the aircraft gets to the runway and the Local Controller issues a new mitigated heading. What that heading is depends on where the aircraft is going. In a Runway 9 operation the north departures will get heading 081 and the south departures will get heading 096. In a Runway 27 operation the north departures will get heading 268 and the south departures will get heading 245. With the exception of the 245 heading; it seems like the headings were randomly picked. The headings can be confusing to pilots and the controller work load is increased with the additional phraseology of issuing departure headings. I recommend discontinuing the use of mitigated headings and go back to using the PHL8 departure.