json_decode returning "NULL" for POST data in WordPress

I tried to JSON.stringify an array to the server like so:

JavaScript:

jQuery.ajax({
    type: 'post',
    url: ajax_url, 
    data: {
       'settings' : JSON.stringify([...]),
       'action'   : [...]
       },
    traditional: true,
    success:function(data) {
        alert("SUCCESS");
    }, 
    error: function(errorThrown){
       console.log(errorThrown); 
   } 
});

But when I tried json_encode it in PHP it returns NULL.

PHP

$param = json_decode($test, true);
var_dump($param); //it retuns NULL

And JSON.stringify array is displayed like this:

{\"uid\":{\"@cdata\":\"6\"},\"board_name\":{\"@cdata\":\"test\"},\"skin\":{\"@cdata\":\"default\"},\"use_comment\":{\"@cdata\":\"\"},\"use_editor\":{\"@cdata\":\"\"},\"created\":{\"@cdata\":\"20160307182421\"}}

What I'm doing wrong?