JSON APIs in PHP, across the internet

I am building a proof of concept for data processing web service, that allows users to get data cleaned, homogenized or otherwise manipulated.

Often users send different types of data in the same batch. After processing, the data must be distributed to different users individually or in pairs, based on the type of data.

The frequency is quite high, expecting data from each user every 30 secs.

I keep going back and forth on the model for implementing this service.

I am leaning towards a POST/POST operation as follows:

1. the user POSTs a batch of data in JSON format to the service on a given URL on my end
2. The service processes the data
3. The service POSTs it to the different recipients of that data on a given URL at their end

But maybe some of you have a better idea or experience you can share with something similar.

Your input is appreciated.