$('#test').click(function(){
   var obj= {
                'key': 'value',
            };
$.ajax({
                url: "../folder/file.php",
                type: "POST",
                data: {
                    'obj' : obj
                },
                dataType: "JSON",
                success:function(data){
                    alert(data);
                }

            });
});

$data = $_POST['obj']; // this is line 1
echo json_encode($data); // this is line 2

var_dump($_POST['myobj']);