Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Confusion
Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Taxiway
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Airport
Turned aircraft onto wrong taxiway at night due to ambiguous signage. (Refer to PHL taxi diagram to visualize the following description). Instead of turning left on P3; I turned left onto L while taxiing east on S.While taxiing east on taxiway S after landing on Runway 9R; ATC initially instructed us to turn left on to Taxiway F and hold short of Runway 9L.  While taxiing on S; the FO (First Officer) contacted operations and learned that our gate was not available. Upon informing ATC that our gate was not open; we received a new clearance to turn left on P3 and hold short of P. Note on the diagram that between P and S near the east end of 9R; there are seven short connector taxiways that are immediately adjacent and parallel to each other. When I received the amended taxi instructions; I had already begun passing the westernmost group of taxiways. While looking at the taxiway signs for P3; I noted that the signs are located equidistant between the taxiways. As a result; it became unclear as to whether the taxiway sign was associated with the taxiway on the left side of the sign; or the right side of the sign. It appeared initially that the taxiway sign for P3 would be on the right side of the aircraft; but after making the turn I noticed that the in-pavement taxiway lights that were now visible further ahead (beyond P) continued across Runway 9L. Since P3 does not cross 9L; I now realized that I had turned one taxiway too soon. We contacted ATC to inform them of my mistake and they told us to remain where we were and to continue to hold short of P; which we did. There was no other traffic in our vicinity. Additional contributing factors include night operations and decreased level of familiarity with PHL due to limited experience there.Upon further review of the taxi diagram I noted that taxiway L was a left-turn only taxiway; while the P3 intersection also would allow a right turn onto S2. Had I noticed this feature during the taxi; I would have been able to confirm that the taxiway that I turned left on could not be P3. The next time that I experience taxiway identification problems; I will stop where I'm at and set the parking brake; and query ATC for clarification.
Air carrier Captain reported turning onto the wrong taxiway due to the ambiguous placement of taxiway signs at PHL airport. Signs are placed equidistant between taxiways; so it is unclear which taxiway is associated with each sign.