A photonic quantization approach to implementing analog-to-digital conversion (ADC) in the optical domain with differential encoding employing a phase modulator and delay-line interferometers (DLIs) is proposed and demonstrated. In the proposed ADC system, the phase-modulated signal is sent to an array of DLIs that have identical time delay difference, but different phase shifts, which are employed to achieve quantization with differential encoding. A proof-of-concept experiment is performed. The quantization of a 10 GHz sinusoidal signal with a bit length of 4 is experimentally demonstrated.