Depends on what you mean by biking. If you are planning to put on rainbow colored spandex and do a 100km race through the streets, Philly kind of sucks for cycling.

If you just want to cruise around leisurely to see the city and use the bike as a sort of transportation and enjoy yourself, Philly is fantastic for cycling. I'm a daily bike commuter in NYC and to me Philly is so much better. The roads are narrower so the cars go slower. Philly is a fantastic city to explore by bike, IMHO the best city for bike adventures in the USA.

You don't have to worry about getting run over by a psycho because traffic is so bad cars don't have a chance to go that fast. Just watch out for cars like when you were 10