What is the best way to send data and receive a response dependent on that data?

Consider the php file used for the request:

 $test = $_POST['test'];

 echo json_encode($test);

I have tried unsucessfully to achieve this with:

 $.ajax({
 type: "POST",
 dataType: "json",
 data: '{test : worked}',
 url: 'ajax/getDude.php',
 success: function(response) {
     alert(response);             
                             }
 });