53 Pa. Stat. § 12635
In case an eligible list consists of less than two names, the appointing officer may, at his option expressed in writing to the commission at the time of the appointment, treat such appointment as temporary, and, if he does so treat it, the commission shall proceed at once to hold an examination and to secure an eligible list as promptly as possible. The appointing officer shall, upon the receipt of the eligible list from the commission, and with sole reference to the relative merit and fitness of the candidates, make an appointment from the two names so certified. After any name has been twice rejected by any one appointing officer for the same or a similar position in favor of others on the same eligible list, the said name shall not again be certified to that appointing officer. When an appointment is made under the provisions of this section, it shall be in the first instance for a probationary period of three months. If during that period the service of that officer or employe is unsatisfactory, the appointing officer shall notify him in writing that he will not be retained in the public service after such three months period. If not so notified, his appointment shall become permanent at the end of the three months probationary period.
53 P.S. § 12635