Air Carrier Captain reported while exiting the runway and turning onto the taxiway suddenly noticing an elevated runway guard light resulting in taking evasive action to avoid hitting the light.

Narrative

A long day of delays; thunderstorms avoidance; and low approaches we show up in PHL after diverting first to ZZZ due weather. The FO makes a nice approach and landing on Runway 27R. As we roll out I take control of the aircraft as the Controller states to take the high speed; which I did. Keep in mind that it's been a long day after midnight dark and rainy. I exit K5 from 27R; and follow the controllers instructions to taxi on K then spot 8 to the ramp. When I made the right turn to join K; I suddenly noticed I was aiming the aircraft for the back of the elevated Runway guard light at K5. I quickly changed course and avoided the K5 Island which contains the Runway guard lights. In this case everything worked ok as I regained my bearings and parked the aircraft safely. I would like to point out that this island is invisible at night in low light conditions like we were experiencing. This island has edge marking and painted green; but there are no lights or reflectors to mark this off to be visible at night. So when I looked over to taxiway K it appeared clear. Who knew PHL has invisible islands in the middle of taxiways. When taking the high speed at K5 following the CL lights; there is no line to follow that takes an aircraft eastbound on K. This in my mind a similar situation as landing CLT landing 18L and exiting on the C6 high speed then double back northbound on C without a line to join C and this is something I do on a regular basis. The difference being there are no obstacles in the CLT; C6 taxiway.My first recommendation is add edge lights to the K5 Island; so pilots can easily see this area when taxing. Second I would add to the Jepp 10-7 pages that PHL uses reflectors in place of edge lighting on most taxiways. I would consider this a threat particularly in low visibility conditions; at night and taxing with aircraft that might have taxi lighting on MEL. I find it interesting other airports spend millions on taxiway edge lighting and PHL just uses reflectors! Also consider the plie of NOTAMS issued at various airports referring to nonstandard signs; markings; faded markings etc. But no mention anywhere at least that I found that states PHL uses reflectors; and some places on the airport have no edge illumination whatsoever.