ASIHTTPRequest向php服务器发送json

I have been trying to post a json object to a PHP script for the last 5 hours. I have read all the docs and it seems the code should work BUT it does not. The request is made and received ok, but I can't access the posted json data or dont even know if its been sent correctly. I keep getting an empty php array, instead of the json data.I have been trying to post a json object to a P