title: Can't retrieve jQuery serialized form data with PHP $_POST[] variable
body: <p>Using Ajax jQuery function <code>$.post()</code>, I'm sending jQuery <code>serialized()</code> form data to a PHP function which successfully receives it in <code>$_POST[]</code> yet individual <code>$_POST['form_field']</code> variables are empty.</p>
  
  <p>In my PHP function, <code>print_r($_POST)</code> successfully displays the form data:</p>
  
  <pre><code>Array ( [action] =&gt; send_message [data] =&gt; modal_name=olivier&amp;modal_email=olivier%40hotmail.com&amp;modal_message=Hello+world )
  </code></pre>
  
  <p>Yet <code>$_POST['modal_name']</code>, <code>$_POST['modal_email']</code> and <code>$_POST['modal_message']</code>
  are empty. Why?</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;form id='contact'&gt;
    &lt;input type='text' name='modal_name' id='modal_name' /&gt;
    &lt;input type='email' name='modal_email' id='modal_email' /&gt;
    &lt;textarea name='modal_message' id='modal_message'&gt;&lt;/textarea&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>$('#contact').on('submit', function(e) {
    e.preventDefault();
    if (modal_name &amp;&amp; modal_email &amp;&amp; modal_message) {
      var data = {
        action: 'send_message',
        data: $(this).serialize()
      };
      $.post(WPaAjax.ajaxurl, data, function(response) {
        $('.modal').append(response);
      });
    }
  });
  </code></pre>