Philly is a REALLY fun and accessible city - I spent a lot of time in college & my 20s there, even lived there, and it's so much fun! Walkable, great mix of big enough to have lots of options/resources/stuff to do without being so big as to be overwhelming. I grew up right outside Philly, but I also spent a lot of time in Boston & NYC, both of which are just too big IMO. It has a wonderful mix of local staples with new fun hip things. Great bars, restaurants, museums & arts options, parks & green space, music, cultural things to do, and universities with tons of resources & events. University City is pretty safe if you're in the right neighborhoods, and while parking is a total nightmare, walking is not! I only felt unsafe in the city exactly once, when I was taking a late train back to NJ by myself (not a smart or safe move!) - otherwise I've walked plenty of places after dark & felt completely safe. It's also WAY cheaper than most East Coast cities - I had an adorable historic 2 bedroom apartment in an antique row home in one of the nicest areas of Southwest Center City (check out Graduate Hospital neighborhood!), and it was only $700 a month split with my roommate.
I'd highly suggest trying to visit before the acceptance deadline if you possibly can - it's TERRIFYING moving to a totally new place & new environment without ever experiencing it. I can say all I want how much fun it is in Philly, but only you know what you like! Happy to recommend places to go & things to see in the city if you do!