If you are a PA resident, all your income is taxable to PA, regardless of what state it was earned in. In some situations, it is also taxable to the other state, but PA will grant a credit for taxes paid to the other state.
However, PA also has reciprocal agreements with some states where the other state will not tax the wages of a PA resident. Off the top of my head, OH, NJ, MD, VA, IN, WV are reciprocal states.
If you provide more specific information, you may bet a more specific answer for your situation.
