Philadelphia was a city in the eastern United States of America on Earth, in the region of Pennsylvania.
In 2032, Francis M. Raymond, the son of Denise C. Raymond and Brent S. Jay Raymond, was born in this city. (TNG: "The Neutral Zone")
This information came from the remastered Clare Raymond family tree. Francis M. Raymond was named after Francis Mulcahy, a character from the dramedy M*A*S*H. This character was from Philadelphia.
In the Bashir 62 holoprogram, Vic Fontaine and Frank Chalmers lived in south Philadelphia. Vic often beat Frankie in games of stickball there, leading to a lifelong rivalry between them.
Philadelphia was known for its cheesesteak sandwiches. Tony Cicci once asked a waiter for a cheesesteak and told him "I don't care if you gotta fly to Philadelphia to get me one". (DS9: "Badda-Bing, Badda-Bang")
In the 24th century there was a Federation starship named the USS Philadelphia, presumably named in honor of the city. (TNG: "Eye of the Beholder")