How to connect multiple sensors to single PLC input?
You more have to make sure 1 sensor does not damage another, 

E.g. if 1 outputs "HIGH" and 1 outputs "LOW" if its a driven output, its essentially a short circuit on the sensor outputs.

If its "Open Collector" outputs, or "Open Emitter" outputs, then they are safe to have in parallel so long as they are all the same type. as you say its a PNP output, I am going to assume Open Emitter, 

I am going to assume the PLC input is measuring voltage, if so you should not have to worry about damaging the PLC input, as it should be a very high resistance, so the current from the outputs would be very small. This is assuming that the PNP output voltage is not higher than the maximum voltage rating for the PLC input.