42 Pa. C.S. § 5930
Except as otherwise provided in this subchapter, in any civil action or proceeding, where any party to a thing or contract in action is dead, or has been adjudged a lunatic and his right thereto or therein has passed, either by his own act or by the act of the law, to a party on the record who represents his interest in the subject in controversy, neither any surviving or remaining party to such thing or contract, nor any other person whose interest shall be adverse to the said right of such deceased or lunatic party, shall be a competent witness to any matter occurring before the death of said party or the adjudication of his lunacy, unless the action or proceeding is by or against the surviving or remaining partners, joint promisors or joint promisees, of such deceased or lunatic party, and the matter occurred between such surviving or remaining partners, joint promisors or joint promisees and the other party on the record, or between such surviving or remaining partners, promisors or promisees and the person having an interest adverse to them, in which case any person may testify to such matters; or, unless the action is a possessory action against several defendants, and one or more of said defendants disclaims of record any title to the premises in controversy at the time the suit was brought and also pays into court the costs accrued at the time of his disclaimer, or gives security therefor as the court in its discretion may direct, in which case such disclaiming defendant shall be a fully competent witness; or, unless the issue or inquiry be devisavit vel non, or be any other issue or inquiry respecting the property of a deceased owner, and the controversy is between parties respectively claiming such property by devolution on the death of such owner, in which case all persons shall be fully competent witnesses.
42 Pa.C.S. § 5930