I'd recommend checking out the website Craigslist. Go to the Philadelphia section and look under Sublets/Temporary for apartments that are available short-term. For neighborhoods, anything that says Center City is most always decent. Specifically, look for any of the following neighborhoods for relatively safe/centrally located/fun places to live:
Art Museum, Bella Vista, Center City, Society Hill, Washington Square, Old City, Rittenhouse Square or Queen Village
Good luck! (Below is an example which might get flagged but it's a good example...completely random, the ad isn't mine)
http://philadelphia.craigslist.org/sub/1030301054.html