This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
Pennsylvania requires front lamp emitting a beam of white light intended to illuminate the rider's path and visible from at least 500 feet to the front. For the rear, red reflector facing to the rear, visible from at least 500 feet when directly in front of lawful low-beam headlamps. a red rear lamp (steady or flashing) visible from 500 feet may be added but does not replace the reflector. an amber reflector is required on each side of the bicycle. Side and/or pedal reflectors are also required. The rule applies whenever it is dark. See 75 Pa. Cons. Stat. § 3507.
The cheapest way to stay legal in Pennsylvania is a USB-rechargeable front white light and a rear red light or reflector that meets the spec above. A $30 light set covers it.
Pennsylvania also requires reflectors beyond the core front and rear lights. New bikes from a US shop will already meet this; if you've stripped the reflectors off, replace them or add reflective tape on the frame and pedals.