The Philadelphia airport parking is a complex issue. The parking rates are very high: 24h hours of parking can cost something between $11 and up to $40. The main reason for this is that there are very high taxes on parking lots around the air port and inside the phl air port itself.
We would recommend looking online and locating the lowest, cheapest parking around. The modern way to do it is just Google “philadelphia parking coupons” and you will be able to get tons of it. Your job is just to pick up the best one. This way you will be able to park your car safely and in a reasonable price.