I have discovered Sonny's Famous steaks by accident. Philadelphia is well known for their Cheese steak sandwich and a good place to eat this while visiting Philadelphia historic district is Sonny's. Sonny is 3 blocks East of the the Liberty Bell pavilion and it's located on 2nd and Market.
Their lunch special is reasonably priced and will give you a steak sandwich, delicious french fries and a soda.
I prefer Sonny's over Gino's Steak which is located in South Philadelphia, but some can disagree with me. The hoagie roll is nice and fresh and you get a great amount of meat on your sandwich. Add some fried onions or your favorite cheese and you have your authentic Philly Steak sandwich.
If you are out partying in Ol' City, you can stop by Sonny's after 2AM for a quick bite.