A320 CREW HAD ALT OVERSHOOT IN P50 CLASS B AIRSPACE.

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FLT WAS BEING RADAR VECTORED TO ENTER DOWNWIND LEG FOR RWY 8 AT PHX ARPT. CLRNC FROM 12000 FT MSL TO 8000 FT MSL WAS ISSUED BY APCH CTL; WITH ADDITIONAL INSTRUCTIONS TO SLOW TO 210 KTS AT 10000 FT BEFORE CONTINUING DSCNT TO 8000 FT. IT IS COMPANY POLICY TO USE AUTOMATION AS MUCH AS POSSIBLE; INCLUDING USE OF AUTOPLT UNTIL ON 3-5 MI FINAL. IN ATTEMPT TO FOLLOW COMPANY POLICY; ALT ON MCP WINDOW WAS SET TO 10000 FT MSL. THE PLAN WAS TO LEVEL AT 10000 FT; SLOW TO 210 KTS; THEN COMMENCE DSCNT TO 8000 FT MSL. ALL WAS GOING AS PLANNED UNTIL DSCNT TO ASSIGNED ALT WAS COMMENCED. I HAD FORGOTTEN WHAT ASSIGNED ALT WAS; AND THOUGHT IT TO BE 6000 FT MSL. I QUERIED FO AND HE AGREED IT WAS 6000 FT. DSCNT TO 6000 FT MSL WAS COMPLETED. APCH CTL ISSUED FREQ CHANGE TO FINAL CTLR. FINAL CTLR QUERIED US AS TO ASSIGNED ALT AND WE REPLIED '6000 FT.' CTLR SAID IT SHOULD HAVE BEEN 8000 FT MSL; AND THEN ASSIGNED US TO MAINTAIN 6000 FT. FLT LANDED UNEVENTFULLY. IN RETROSPECT; I WAS VERY FAMILIAR WITH ASSIGNED ALTS WHEN ENTERING THE PHX TRACON AREA. DURING THE DSCNT TO 6000 FT; I KEPT ON THINKING HOW UNUSUAL THIS ALT ASSIGNMENT WAS. I SHOULD HAVE QUERIED THE CTLR AS WELL AS THE FO AS TO ALT ASSIGNMENT. HOWEVER; THE AIRSPACE AND RADIO FREQ WERE QUITE CONGESTED. I THOUGHT THAT THE FO'S AFFIRMATIVE AGREEMENT TO 6000 FT WAS ENOUGH. I WAS WRONG. AS ANOTHER FACTOR; FATIGUE WAS ALSO PRESENT. IT WAS THE LAST LEG OF A 4-DAY TRIP AND WAS FATIGUED. SUPPLEMENTAL INFO FROM ACN 493660: DSNDING ON THE FOSSL ARR INTO PHX; OUR FLT 'ACR ABCD' MAY HAVE HAD AN ALTDEV. WHILE LEVEL AT 6000 FT MSL; THE CTLR STATED SOMETHING LIKE 'I THINK YOU WERE ONLY CLRED TO 8000 FT.' NEITHER THE CAPT NOR I WERE SURE IF WE HAD ERRED OR NOT; BUT I DO RECALL BEING GIVEN 1 XMISSION FROM APCH THAT CONTAINED AT LEAST 3 SEPARATE INSTRUCTIONS -- POSSIBLY A HDG; SPD; INTERCEPT INSTRUCTIONS; AND AN ALT CLRNC.