Did you try google:

PA resident working in DE

"Two of Pennsylvania's neighboring states do not offer income tax reciprocity: Delaware and New York. This means, for example, a Pennsylvania resident working in one of those states must file a return in that state, pay the tax, and then take a credit on his or her Pennsylvania return."

https://www.picpa.org/keep-informed/pages/taxes-for-living-and-working-in-different-states

You should bookmark this, as it becomes one of the basic resources you can use again and again.