A320 FLT CREW HAS ACFT ROLL FORWARD AFTER STOPPING AT GATE TO DEPLANE.

Narrative

AFTER A VERY LONG AND TIRING DUTY DAY (AIRPLANE SWAP AND A LONG SIT) AND OVER 7 HRS OF FLYING WE ARRIVED IN PHX. OUR GATE WAS CHANGED AND EVEN WITH MY 10-8 AND 10-9 CHARTS OUT; IT WAS DIFFICULT TAXIING WITH THE CONSTRUCTION. AS I MANEUVERED THE ACFT TOWARD THE GATE; THE INSTRUCTIONS FROM THE MARSHALLER WERE NOT THE MOST PRECISE INSTRUCTIONS (CONFUSION AS TO WHICH LEAD-IN LINE TO USE; UNUSUAL GUIDANCE SIGNALS; AND A VERY ABRUPT SIGNAL TO STOP. I FOUND OUT LATER HE WAS NEW). IN MY OPINION; THE GATE AREA WAS POORLY LIT. I STOPPED THE ACFT AT THE GATE WHEN THE MARSHALLER SIGNALED ME; AND THOUGHT I SET THE PARKING BRAKE. I WENT ABOUT MY POSTFLT DUTIES WHILE WAITING FOR THE GND PWR TO BE PLUGGED IN. THE FO SUDDENLY SAID; 'WE'RE MOVING' AS I ALSO NOTICED THE ACFT WAS ROLLING FORWARD. I STEPPED ON THE TOE BRAKES; RESET PARKING BRAKE AND DOUBLECHKED THE YELLOW PRESSURE GAUGE -- IT OPERATED NORMALLY. I TALKED TO GND PERSONNEL WHO RPTED NO INJURIES TO ANYONE AND NO DAMAGE TO ACT. I SPOKE TO THE CUSTOMER SVC REPRESENTATIVE; TO CONFIRM THAT NO ONE; AND NOTHING WAS HURT OR DAMAGED. SINCE I TRULY THOUGHT I SET THE PARKING BRAKE; I THOUGHT IT WAS ONE OF THE 'AIRBUS ELECTRONIC GLITCHES' THAT CAUSED THE ACFT TO ROLL.