从php中的URL获取json数据的问题

Hi all am Working on web service for mobile applications in my case the data from the mobile will be received in the json format the problem is i can't decode the json data only i can get the post values of it

json data send from mobile

test:{"username":"abc1234","emailid":"abc@asd.com","password":"abc123","transactionpassword":"abc1234","phone":"1234567890","secretquestion":"What is the name of my pet?","secretanswer":"Tom"}

the following array obtained by

print_r($_POST);
    Array
    (
        [------WebKitFormBoundaryXb9aAj9gBd1dA41z
    Content-Disposition:_form-data;_name] => "test"

    {"username":"abc1234","emailid":"abc@asd.com","password":"abc123","transactionpassword":"abc1234","phone":"1234567890","secretquestion":"What is the name of my pet?","secretanswer":"Tom"}
    ------WebKitFormBoundaryXb9aAj9gBd1dA41z--

    )

while i tried to decode it no results were shown

print_r(json_decode($_POST));

empty

Help me to decode the json data in php

It looks like you should fetch actual POST param first.

json_decode($_POST['test'])