No need for analog.

Tie the cathode of the photo diode to the PIC pin with a 10M ohm pull up. The anode of the photo diode to zero(ground). When no light is detected the PIC pin will go LOW.

Used an IR diode for a dedicated light source.

You will have to shield from ambient light to keep from having false readings.

This is very similar to how a mouse works. Very fast and reliable.

I have used this setup to detect parts moving in excess of 6 inches per second.