42 Pa.C.S. § 7540: Parties.

(a) General rule.--When declaratory relief is sought, all persons shall be made parties who have or claim any interest which would be affected by the declaration, and no declaration shall prejudice the rights of persons not parties to the proceeding. In any proceeding which involves the validity of a municipal ordinance or franchise, such municipality shall be made a party and shall be entitled to be heard.

(b) Tax matters.--In any proceeding which involves the effect of any asserted legal relation, status, right, or privilege upon the determination of any tax, the appropriate taxing authority shall be served with a copy of the proceeding, but if such taxing authority does not enter its appearance, the requirements of this section shall nevertheless be satisfied if the court considers that the interests of the taxing authority are adequately represented.