53 Pa. Stat. § 15914
When a transcript of the proceedings has been recorded as provided in section 12 of this act and no action has been brought asserting the invalidity thereof within the time specified in section 13 of this act or when the proceedings have been approved finally by the court, then, notwithstanding any defect or error in such proceedings, the validity of the proceedings, the right of the city to issue the bonds authorized thereby, the lawful nature of the purpose for which the bonds are issued, and the validity and enforceability of the bonds in accordance with their terms shall not thereafter be inquired into judicially, in equity, at law or by civil or criminal proceedings, or otherwise, either directly or collaterally except where a constitutional question is involved: Provided, however, That nothing herein contained shall entitle any person knowingly participating in the sale or issuance of bonds hereunder in violation of this act to enforce the obligations of such bonds or shall relieve such person from civil or criminal liability for such participation or for wilfully false or fraudulent statements made in the documents constituting the transcript of proceedings or in the bonds.
53 P.S. § 15914