Guestbook med captcha

Jeg bruger max guestbook på mit site.
Men jeg bliver overdynget med spam fra spambots.

Hvordan kan jeg insætte et simpel captcha system i min guestbook.


Her min kode:

<?php
class maxGuestbook{
  var $messageDir = 'messages';
  var $dateFormat = 'Y-m-d g:i:s A';
  var $itemsPerPage = 9;
  var $messageList;
 
function processGuestbook(){
  if (isset($_POST['submit'])) {
      $this->insertMessage();
  }
  $page = isset($_GET['page']) ? $_GET['page'] : 1;
 
  $this->displayGuestbook($page);
}
 
function getMessageList(){
   
  $this->messageList = array();
 
    // Open the actual directory
    if ($handle = @opendir($this->messageDir)) {
        // Read all file from the actual directory
        while ($file = readdir($handle))  {
            if (!is_dir($file)) {
              $this->messageList[] = $file;
          }
        }
    }   
   
    rsort($this->messageList);
   
    return $this->messageList;
} 

function displayGuestbook($page=1){
      $list = $this->getMessageList();
      //echo "<center><a href='add.php'>Leave a message</a></center>";
      echo "<table class='newsList'>";
     
      //Get start point and end point
      $startItem = ($page-1)*$this->itemsPerPage;
      if (($startItem + $this->itemsPerPage) > sizeof($list)) $endItem = sizeof($list);
      else $endItem = $startItem + $this->itemsPerPage;
     
      for ($i=$startItem;$i<$endItem;$i++){
        //foreach ($list as $value) {
        $value = $list[$i];
          $data = file($this->messageDir.DIRECTORY_SEPARATOR.$value);
          $name  = trim($data[0]);
          $email = trim($data[1]);
        $submitDate = trim($data[2]);   
        unset ($data['0']);
        unset ($data['1']);
        unset ($data['2']);
         
        $content = "";
        foreach ($data as $value) {
              $content .= $value;
        }
         
          echo "<tr id=\"class-name\"><th align='left'>$name</th></tr>";
          echo "<tr><td>".nl2br(htmlspecialchars($content))."<br/></td></tr>";
      }
      echo "</table>";
      if (sizeof($list) == 0){
        echo "<center><p>Be the first to write a comment!</p><p>&nbsp;</p></center>";
      }
      // Create pagination
      if (sizeof($list) > $this->itemsPerPage){
        echo "<div id=\"navigation\">";
        if ($startItem == 0) {
            if ($endItem < sizeof($list)){
              echo "<div id=\"nright\"><a href=\"".$_SERVER['PHP_SELF']."?page=".($page+1)."\" >Next &raquo;</a></div>";
            } else {
              // Nothing to display
            }
        } else {
            if ($endItem < sizeof($list)){
              echo "<div id=\"nleft\"><a href=\"".$_SERVER['PHP_SELF']."?page=".($page-1)."\" >&laquo; Prev</a></div>";
              echo "<div id=\"nright\"><a href=\"".$_SERVER['PHP_SELF']."?page=".($page+1)."\" >Next &raquo;</a></div>";
            } else {
              echo "<div id=\"nleft\"><a href=\"".$_SERVER['PHP_SELF']."?page=".($page-1)."\" >&laquo; Prev</a></div>";
            }
        }
       
        echo "<br/></div><br/>";
      }
      echo "<hr />";
      $this->displayAddForm();
}

function displayAddForm(){
?> 
  <form class="iform" action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    Name:<br/>
    <input type="text" name="name" size="30" style="width: 200px"/><br/><br/>
    Your message:<br/>
    <textarea name="message" rows="7" style="width: 90%" wrap="hard" ></textarea><br>
    <br/>
    <center><input type="submit" name="submit" value="Save" /><br/>
    <br/>
    </center>
  </form>
 
<?php 
}

function insertMessage(){
  $name  = isset($_POST['name']) ? $_POST['name'] : 'Anonymous';
  $email  = isset($_POST['email']) ? $_POST['email'] : '';
  $submitDate  = date($this->dateFormat);
  $content = isset($_POST['message']) ? $_POST['message'] : '';
 
  if (trim($name) == '') $name = 'Anonymous';
  if (strlen($content)<5) {
      exit();
  }
 
  $filename = date('YmdHis');
  if (!file_exists($this->messageDir)){
      mkdir($this->messageDir);
  }
  $f = fopen($this->messageDir.DIRECTORY_SEPARATOR.$filename.".txt","w+");       
  fwrite($f,$name."\n");
  fwrite($f,$email."\n");
  fwrite($f,$submitDate."\n");
  fwrite($f,$content."\n");
  fclose($f);
 
}
}
?>