The city of Philadelphia is well-equipped with public transport, most of it run by SEPTA (Southeastern Pennsylvania Transportation Authority). There is a subway system, but this does not serve the city centre. It's mainly useful for accessing the extremities of the city.
Good, reliable buses are the main standby, particularly the purple PHLASH bus service designed for visitors. The PHLASH route loops through downtown, covering numerous major attractions near about 22 stops.
PHLASH buses operate daily between May and September, between about 10am and 6pm with a more limited service between September and December. SEPTA buses and commuter trains cover numerous fixed routes through the rest of the city and suburbs.
Taxis are plentiful in the city centre but scarcer elsewhere, particularly at night. It is possible to rent cars, but not strictly necessary. Having a vehicle can be useful for those wanting to explore further afield. Uber, Lyft, and other ride-hailing app services are also available.