61 Pa. Code § 113.12
Every employer required to deduct and withhold tax from compensation of an employe shall be liable for the payment of such tax whether or not it is collected from the employee by the employer. For purposes of assessment and collection, any amount required to be withheld and remitted to the Department and additions to tax, penalties, and interest with respect thereto shall be considered the tax of the employer. If the employer fails to withhold taxes and thereafter the tax is paid, the tax which was required to be withheld shall not be collected from the employer. Such payment does not relieve the employer from liability for penalties, interest, or additions to the tax applicable with respect to such failure to withhold. Any amount of tax withheld shall constitute a special fund in trust for the Department, which shall be enforceable against the employer, his representative or any other person receiving any part of the fund.
61 Pa. Code § 113.12