Hmm. I don't think Philadelphia has any local foods.
Okay, now that I've surgically removed my tongue from my cheek...
The Cheesesteak. Description found here:
http://www.etiquettehell.com/smf/index.php?topic=90832.msg2403785#msg2403785The soft pretzel. Jury's out on whether Philly did it first, but they do it best (and most... a common job for preteens used to be hawking pretzels. You'd load a box/basket/milk crate up with pretzels, 11 for $1. Then you'd sell them for $0.25. On a typical weekend, or any day in the summer, you could hear the kids calling out "Fresh pretzels... all ready... fresh pretzels. On a decent day, you could make $5 or $10. My personal best was $25 at a streetside flea market. I just circled the block and the money poured in.)
The hoagie. It's got its roots in Philly as much as anywhere. You might also know it as a po' boy, a sub, or a hero, but a hoagie is a long Italian roll, upon which is lettuce, tomato, onions, oil, meats, and cheese.
The Tastykake. This is a company, but around Philly, you'd be hard pressed to find much int he way of Hostess goods, Tastykake rules supreme. Pies, cupcakes, angel food cakes... all soft and delicious. A Philly lunch might consist of a hoagie, a Tastykake, and a Frank's soda to drink. Yes, it's fattening, but we don't eat like that every day.

(Edited to format for easier reading)