FLC DSNDS TO WRONG ALT AS PER CLRNC TO ANOTHER AIRPLANE WITH A SIMILAR CALL SIGN.

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WHILE OPERATING ACR XBCD AND RECEIVING RADAR VECTORS FOR A CONVERGING RWY 9R ILS AT KPHL; WE WERE ISSUED THE FOLLOWING CLRNC: 'ACR XBCD; TURN TO 210 DEGS; DSND TO 6000 FT.' OUR REPLY WAS: '210 DEG HDG; LEAVING 8000 FOR 6000; ACR XBCD.' I; THE PNF THEN CHANGED THE ALT ALERTER TO 6000 FT; POINTED TO IT; AND WAITED FOR THE PF TO ALSO POINT TO THE 6000 FT IN THE WINDOW OF THE ALERTER (WHICH HE DID). SHORTLY THEREAFTER; WHILE IN OUR DSCNT TO 6000 FT; WE WERE NOTIFIED BY APCH: 'ACR XBCD; YOUR ASSIGNED ALT IS 8000 FT' (INTERFERENCE). THE PNF THEN RESPONDED: 'ACR XBCD WAS GIVEN A HDG OF 210 DEGS AND A DSCNT TO 6000 FT; WHICH WE READ BACK.' THERE WAS A LONG PERIOD OF SILENCE (DURING WHICH EVERYONE WAS LOOKING AT TCASII; I'M SURE) FOLLOWED BY THE CTLR STATING THERE WAS AN ACR XDCD ON FREQ WITH A SIMILAR SOUNDING CALL SIGN. TO PRECLUDE ANY FURTHER ERROR; WE THEN RESPONDED WITH 'ACR EX BE SEE DEE VERSUS 'ACR XBCD.' FORTUNATELY; THERE WAS NO CONFLICT. BECAUSE BOTH PLTS HEARD 'ACR XBCD..' AND BOTH PLTS VERIFIED THE ASSIGNED ALT OF 6000 FT BY POINTING TO IT IN THE ALERTER WINDOW; I'M CERTAIN THE CTLR ISSUED US A CLRNC HE MEANT FOR ACR XDCD; AND DIDN'T CATCH HIS ERROR IN OUR READBACK. (TYPICAL PLT RESPONSE -- BLAME THE CTLR; RIGHT?!) REGARDLESS; I'M VERY PLEASED WE HAVE OUR ALT VERIFICATION PROC AT ACR AND WOULD CERTAINLY ENCOURAGE OTHER OPERATORS TO ADOPT IT EVEN IF IT SEEMS SILLY AT FIRST.