Can physical custody parent leave the state if the other parent has already left? PENNSYLVANIA

Here is the general attitude by the Pa courts:
When one parent desires to relocate - whether to another part of the state, outside of Pennsylvania, or outside the U.S. - the custody and visitation rights of the other parent must be considered.

In Pennsylvania, family courts will always put the "best interests" of the child first, and in a move away situation, they will examine the "Gruber factors" when arriving at its decision. These factors include:

Potential advantages of the proposed move: This includes the likelihood that the move would substantially improve the custodial parent and children's quality of life and that the move is not the result of the non-custodial parent's momentary whim. Is the parent moving because of a better job? Is the parent moving to be by family in order for the child to have the support of an extended family?
The integrity of the custodial and non-custodial parents' motives in seeking the move or trying to prevent it: In seeking the move, is the parent trying to limit or interfere with the other parent's visitation rights? In trying to prevent the move, is the parent doing it out of spite or doing it out of love for the child?
The availability of realistic, alternative visitation arrangements that would sufficiently foster an ongoing relationship between the non-custodial parent and children: Will the parent be able to see the child as in the past? Is the parent proposing to move the child to California or to Maryland? The distance makes a difference.
If you want to move from the jurisdiction of your child custody order, you must notify the other parent of your intent to move. And you must file with the court.

If you have been told that your child's other parent wants to relocate your child to another part of the state or country, you have the right to go to court to prevent the child's removal from the state.

Courts Pennsylvania have found that insignificant geographical changes, such relocation to a nearby city, generally will not constitute a substantial change in circumstances that warrant modification, meaning a change in ustody or a modification of the parenting plan.

Now, how all this applies to your situation. Since, at this point in time, the natural father has not elected to pursue a parenating plan ( visitation), nor have you elected at this point in time to pursue child support ( a mistake , I think, on bopth of your parts) but nontheless, there are no existing orders to violate. If, the father were to protest, collaerally ( after the moves), he certainly could not ask the court for a contempt on you, since there no orders. The fact he" moved away" without asserting his rights, could be a per se defense . The fact the statues exist indicates that the courts are expecting both parents to have a de facto and dejure interest in the parenting of the child.
In your case, where it is obvious you can independently support your child, factored with the fact the father has shown little interest, pragmatically, that the government is unlikely to get involved in the life of these parties, at least at the present time. I suppose that would mean, you can do as you please, until someone tels you you can't. Pa is among the most liberal of states when it comes to "move aways", anyway.