After cleared for a visual approach the flight crew of a B737-700 descended below the floor of the Class B at PHX. They were alerted by Approach Control and climbed to comply.

Narrative

Evening VMC; unlimited visibility; both pilots had airport in sight; and informed approach. Approach gave us a 230 heading; cleared for a visual approach 26. I recall being at or above 5000 MSL at time of clearance and the Captain put 3;000 in the ALT window; and asked if I saw it and concurred. I looked down at the map display; saw the assigned heading/course would take us just outside the fix from which the Captain had extended the final course. It had an at/above 3000 restriction; and; given our altitude and distance from the fix I figured that would work out fine. I went heads out; looking for traffic. Next; I made the '4000 for 3000' call and; shortly thereafter; approach called to let us know we were below the Class B floor of 4000. I acknowledged and the Captain climbed back up to 4000. Rest of the approach was normal. We could have referred to the Class B chart though--when off segment; and going 210 KTS--by the time we determined our position relative to the chart; we might have stayed too high too long for a stable approach. Given our off-segment position; and ATC's familiarity with the local area; ATC could have helped by stating 'maintain 4;000 FT until X miles from the runway; cleared for the visual'.