Zero crossing detector output to another optocoupler
Figure 1. Here is how you would wire it for a PNP input but it probably isn't going to be much good. For NPN connect the top terminal to the input and the bottom terminal to ground.
It's not clear what you are trying to do but most PLCs have a scan time in the order of 2 to 20 ms. 50 Hz mains has a zero-cross every 10 ms and 60 Hz every 8.33 ms. As a result your PLC probably won't detect the zero-cross unless you are using a high-speed or interrupt input.