Leolow,
I agree with Danpreps. I think your talking about light activated sensor. Now, you did not not mention what kind of PLC processor you are using. Anyway, just like any sensor it should go to the input module of your PLC. Just make it sure the ouptut of your light activated sensor is more or less the same with the other input signals of your PLC ( ex. 24 VDC, PNP ). Otherwise, you can use an interface relay.