Is there a digital input that registers light? / Can only ADC pins register signals from an LDR?
The LDR connected to a comparator (or opamp in comparator mode) can provide a simple make or break signal. On/Off. The below circuit is a IR Led and Photodiode Proximity sensor, and the LM358 compares the analog voltage at R2 with a Potentiometer RV1. 



You could go with a better method, a digital ultrasonic sensor. It will provide a pulsed digital signal, and be much less susceptible to light reflections or anything, while having a greater distance. Bonus, it will give you a definitive distance.