Philadelphia's voter turnout isn't particularly high (in any year). It's generally a lot lower than the neighoring suburban counties and lower than the state as whole. It does vote around 80% for Democrats. But, you know, it's 40%+ black and has been a Democratic machine city forever.
The only reason to think there's voter fraud (any, but certainly of the kind that would influence a statewide election result) in Philly is if "black people voting" is voter fraud.
And as with many things, the "eyes" are always on the city. If you really had a plan for massive ballot stuffing for a statewide race, you'd do it elsewhere.