72 Pa. Stat. § 710
Every corporation or association, and every individual liable by law to pay to the Commonwealth a tax upon the gross receipts derived from any transportation, traffic, or other business, done wholly within this Commonwealth, shall transmit to the Department of Revenue, on or before the fifteenth day of April of each year, a statement of the amount of such corporation's, association's, or individual's gross receipts from all sources, and gross receipts from business done wholly within the State, during the period of twelve months immediately preceding the first day of January of each year, with such other relevant information as the Department of Revenue may require. Every such corporation, association, and individual, at the time of making every report required by this section, shall compute and pay to the department the tax due the Commonwealth upon its gross receipts for the period for which such report is made.
72 P.S. § 710