Passing a JavaScript Array To PHP Through JQuery $.post

This piece of code gets all the form variables and sends them via AJAX to the PHP script. But I want the calculated results from the PHP script that is being returned to the javascript via a JSON encoded array to be in the form of "post":{"uname":"someNamefromForm","email":"someEmail","fname":"namefromtheform","lname":"lastnamefromform" }... The output I'm getting now is "uname=e&email=e&fname=e&lname=euname".. This is the JSON array I want to displayed at the bottom of the page for debugging purposes. Can someone tell me how to format it please

This is my HTML form

  <div id="wrapper">
      <h2> Validation with AJAX,JQuery,JSON and PHP</h2>
          <div class="form-container">    
              <span id="ajax-message"></span>

              <form id="ajax-form" onsubmit="return false;">
                  <p class="legend">All fields marked with an asterisk are required.</p>

                  <fieldset>
                      <legend>User Details</legend>
                      <div>
                        <label for="uname">Username <em>*</em></label> 
                        <input id="uname" type="text" name="uname" value=""  />
                      </div>
                      <div>
                        <label for="email">Email Address <em>*</em></label> 
                        <input id="email" type="text" name="email" value="" />
                      </div>
                      <div>
                        <label for="fname" class="error">First Name <em>*</em></label> 
                        <input id="fname" type="text" name="fname" value="" size="50" class="error"/>
                        <p class='note'>All error message go here </p> 
                      </div>
                      <div>
                        <label for="lname">Last Name <em>*</em></label> 
                        <input id="lname" type="text" name="lname" value="" size="50" />
                      </div>
                  </fieldset>

                  <div class="buttonrow">
                      <input type="submit" value="Submit This Form via AJAX" class="button" />  
                      <input type="button" value="Start Again" class="button" />
                      <a >Refresh this Page</a>
                  </div>
              </form>
          </div> 
      <h3>JSON Array</h3>
      <pre id='debug'></pre>
  </div>

This is my javascript

$("#ajax-form").submit(function(){
          var variableToSend = $(this).serialize();
          $.post(
            'ajaxformval_post.php', 
            {variable: variableToSend}, 
            function(data){$("#debug").html(data)},
            "json"
          );
      })

This is the php

<?php
    $variable = $_POST['variable'];

    echo json_encode($variable);

    /*$json = array(
        'booleanFlag' => TRUE,
        'someText' => "AJAX should be renamed AJAJ",
        'anArrayOfData' => array('name' => 'Mickey', 'ears' => 'very Big') 
    );*/

 ?>