The doors open inwards, so let's assume that you have a door that is 24" x 48" (an airplane door is a bit bigger) you would have 1,152 sq inches of surface area. Now if there is just a 1 PSI difference between inside and outside (inside being higher of course), then you would need to exert 1,152 pounds of force on the door to get it to move inward. This is one of the factors that led to the deaths in Apollo 1, they had an inward opening door and were conducting the test with 19.7 PSI or so and it would have taken an elephant standing on the door to get it to open.
At cruising altitude (39,000' or so) the difference between inside and outside is over 8 PSI. So going back to the 24" x 48" with 1,152 sq inches, it would take over 4 tons of force to open the door inward. Muhammed himself couldn't pull that off.
drat...I got interupted by a phone call...hence your explanation beat mine...:(