jQuery Grid Plugin - jqGrid

First of all hello to everybody and sorry for my english !

I try to use jqgrid 3.8.2 and I would like to use a multi search toolbar:

these are some setting that I use for my grid

.......

url:'json.php',

mtype: 'GET',

datatype: 'json',
.......... 

If I well understand, when I press the "search" button, a string is send to to server, and my file (json.php) should elaborate the string.

I'm at the begin and I don't konw how to elaborate a sting like in the sample wiki...

{"groupOp":"AND",
     "rules":[
       {"field":"invdate","op":"ge","data":"2007-10-06"},
       {"field":"invdate","op":"le","data":"2007-10-20"}, 
       {"field":"name","op":"bw","data":"Client 3"}
      ]
    }

where can I find any example where is explain how to process the string in php ?!?

Thank You for everything !

Andrea