Guess it's most dangerous in certain 'hoods.
A lot of the old buildings downtown are being converted into classy high end condos and people with money are moving in so you know they're going to make sure it's a safe place to live.
However north only a few blocks away from Center Philly is a real ghetto full of drugs and crime, and to the west and south to lesser extents.
I used to cop drugs and hang out all night in these places. Been mugged and had guns pointed at me more than a few times. Those were the days :)