65 Pa. Stat. § 201
In order to extent to employes, except those hereinafter excluded from the provisions of the act, of the Commonwealth and its political subdivisions and of the instrumentalities of either and to the dependents and survivors of such employes, the basic protection accorded to others by the old-age and survivors insurance system embodied in the Social Security Act, it is hereby declared to be the policy of the General Assembly, subject to the limitations of this act, that such steps be taken as to provide such protection to employes of the Commonwealth and its political subdivisions and to the instrumentalities of either on as broad a basis as is permitted under the Federal Social Security Act. It is also the policy of the General Assembly that the protection afforded employes, in positions covered by a retirement system established by the Commonwealth or a political subdivision on the date an agreement under this act is made applicable to service performed in such positions or receiving periodic benefits under such retirement system at such time, will not be impaired as a result of making the agreement so applicable or as a result of legislative enactment in anticipation thereof.
65 P.S. § 201