title: Contact Form 7: wpcf7_form_response_output filter
body: <p>May I ask how is the wpcf7_form_response_output filter (found in includes/contact-form.php) use?</p>
  
  <p>The below is what I had tried but to no avail. </p>
  
  <p>============== <strong>Case 1</strong> ============</p>
  
  <p><strong>Function 1:</strong></p>
  
  <pre><code>$wpcf7-&gt;skip_mail = true;
  $content = __('You have already subscribed to our Events.',CURRENT_THEME);
  $class = 'wpcf7-validation-errors';
  custom_wpcf7_form_response_output_filter('', $class, $content, '');
  </code></pre>
  
  <p><strong>Function 2</strong></p>
  
  <pre><code>if(!function_exists('custom_wpcf7_form_response_output_filter')){
      function custom_wpcf7_form_response_output_filter($output, $class, $content, $this){
          return '&lt;div class="' . $class . ' "&gt;' . $content . '&lt;/div&gt;';
      }
      add_filter( 'wpcf7_form_response_output', 'custom_wpcf7_form_response_output_filter', 10, 4);
  }
  </code></pre>
  
  <p>========== <strong>Case 2</strong> ==============</p>
  
  <pre><code>$wpcf7-&gt;skip_mail = true;
  $class = 'wpcf7-validation-errors';
  $error = __('You have already subscribed to our Events.',CURRENT_THEME);
  $output = '&lt;div class="' . $class . ' "&gt;' . esc_html( $error ) . '&lt;/div&gt;';
  $omsg = 'return "'. $output .'";';
  add_filter('wpcf7_form_response_output', create_function('$a', $omsg));
  </code></pre>
  
  <p>Case 2: Another method which does not work as well. 500 Internal Server Error when used.</p>
  
  <p>Any help here?</p>