Commercial airplane doors are usually plug doors that open inward (although some are then rotated and pushed outward to get them out of the way). In this way they cannot be opened in flight (except for a certain cargo door on the old DC-10 which caused some problems way back when).
This is correct for the make and model of the aircraft. According to the photo in the news article, the plane where the incident occurred is a Boeing 757, which has plug type doors that open inward first, then swing outward. All 717s, 737s, 747s (main deck), 757s and Boeing/MD aircraft have these manually operated doors. These doors can't be opened inflight because of cabin pressurization putting force on the doors.
Doors that push outward and slide to the side of the fuselage are on all Airbus aircraft and the Boeing 777 and 787. These doors have additional safety latches which prevent the door from opening inflight. Also, it is next to impossible to push out and slide these doors to the side in the forward direction while the plane is flying at 500 mph.