Alcohol can be brought from most duty-free shops at airports when you land and when your depart, but are you allowed to drink while on the plane? And is it very expensive?
Drinking alcohol on commercial flights has been a common practice seen the airline industry began. Of course, when flights began they were so expensive that people expected the highest high in luxury and customer service. When taking flights from Denver to Miami back in those days you could usually drink as much as you liked, even if you were not a first-class passenger. Some things have now changed with airlines restricted the amount of free refreshments that they provide to their customers while on board the airplane.
Following customers becoming drunk and occasionally abusive cabin crews have had to be trained in seeing the signs of someone becoming unruly, and stop giving them drinks before things get out of hand. As a result on flights from Manchester to Kuala Lumpur for example, you may not be able to drink as much as you'd like. This is due to the safety of the cabin crew and other passengers being just as important as keeping one passenger happy and provided with drinks.
So you should expect domestic flights from Dallas to Philadelphia and other similar routes to provide you with exactly the service that you expect from a corporation trying to keep its passengers happy. But when you travel to different culture, especially when flying within the Middle East, you may not be able to get served alcohol. This is due to many of the local people following faiths that forbid the consumption of alcohol and other drugs and as such the airlines prefer not to stock any alcohol at all on board their planes. You can find out if they will serve alcohol by calling before you book your tickets.