The peak intensity of cavity solitons, forming in optically injected vertical-cavity, surface-emitting lasers, is very sensitive to any small variation in the amplitude of the so-called injected beam. The results of simulations show that the time trace of the cavity soliton peak intensity closely depends on the shape, amplitude, and modulation frequency of the injected beam. This fact led to the use of the injected signal in a vertical-cavity, surface-emitting laser to measure the relative phase of two optical signals. Amplification of the input signal variations at the peak of a cavity soliton was used to design a phase detector which can find applications in optical differential phase-shift-keying demodulation systems. It is shown that the proposed scheme can convert, at its best contrast, a 71.43 Gb/s phase-keyed to an intensity-keyed signal.