title: jQuery AJAX response always returns blank message
body: <p>Hi I'am trying to make HTML form, and need to validate it before executing the form action. But the AJAX  respond always returns a blank message?</p>
  
  <pre><code>$(function(){
      $("#ajax-payment-form input[type='submit']").click(function(e) {
  
          // Prevent form submission
          e.preventDefault();
  
          // Serialize data, make AJAX call
          var str = $(this).serialize();
          $.ajax({
              type: "POST",
              url: templateDir+"/payment_form/payment_process.php",
              data: str,
              context: this
          }).done(function(msg) {
  
              if(msg == 'OK') {
                   console.log('Validation passed, will submit form');
                   $(this).closest('form').submit();
              } else {
                   console.log(msg);
              }
  
          }).fail(function() {
              // Catch ajax errors here
              console.log('AJAX error');
          });
      });                                                         
  });
  </code></pre>
  
  <p>PHP:</p>
  
  <pre><code>$post = (!empty($_POST)) ? true : false;
  if ($post) {
    $orderid = stripslashes($_POST['orderid']);
    $amount = stripslashes($_POST['amount']);
    $betingelser = stripslashes($_POST['betingelser']);
    $error = ''; // Check ordreid
    if (!$orderid) {
      $error. = 'Venligst indtast dit ordreid.&lt;br /&gt;';
    } // Check amount
    if (!$amount) {
      $error. = 'Venligst indtast et beløb.&lt;br /&gt;';
    }
    if (!$error) {
      echo 'OK';
    } else {
      echo '&lt;div class="notification_error"&gt;'.$error.
      '&lt;/div&gt;';
    }
  }
  </code></pre>
  
  <p>Can anyone tell me what wrong? </p>