CPR X HAD NEAR GND COLLISION WITH ACR Y DURING FOG CONDITIONS.

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PORTIONS OF PHL WERE COVERED IN HVY GND FOG! DURING ROLL OUT PHL TWR ADVISED CPR X TAKE ANY L TURN TO CLR RWY 9R AND CPU CLR OF RWY 9R. WE COMPLIED! TWR ADVISED US TO TAXI EBOUND ON 'S' TXWY TO 'E.' TAKE L ON 'E' AND HOLD SHORT OF RWY 9L AND REMAIN THIS FREQ! I VERIFIED OUR LOCATION AND DIRECTION WE WERE TO PROCEED BEFORE STARTING OUR TAXI! DURING THE TAXI WE CAME NOSE TO NOSE WITH AN ACR Y PAX JET! WE BOTH STOPPED AND EVENTUALLY WE TURNED OUR ACFT AROUND AND BACK TAXIED WBOUND ON 'S' TO TURN OFF ON TXWY 'Z' TO ALLOW Y TO TAXI BY ALONG WITH ANOTHER AIRBUS! ONCE THEY WERE CLR; WE TAXIED TO PARKING WITHOUT FURTHER INCIDENT! VISIBILITY ON THE GND AT THE MID POINT OF TXWY 'S' WAS 300-400 FT AT BEST! ALTHOUGH WE COMPLIED WITH THE ATC CLRNC; TWR MUST HAVE THOUGHT THE 2 PAX JETS WERE FURTHER W ON TXWY 'S.' ALSO HE MUST HAVE THOUGHT X ROLLOUT WAS LONGER AND THAT WE WERE E OF THE 2 PAX JETS. A CALL FROM US THAT WE WERE CLR OF RWY 9R AT XXX WOULD HAVE HELPED THE TWR A LOT; BUT NEITHER ONE OF US SAW SIGNS FOR WHAT WE PRESUME WAS THE TURN OFF AT 'S6' TXWY CLRING RWY 9R.