Inside your form

   <input type="text" name="validator" id="validator" size="4" />
   <img src="random.php" alt="CAPTCHA image" width="60" height="20" vspace="1" align="top" />

   and test the value of the "validator" form field like:

   if (!empty($_POST['validator']) && $_POST['validator'] == $_SESSION['rand_code'])
   {
       // process your form here at least destroy the session
       unset($_SESSION['rand_code']);
   }

<?PHP
  // save this code in your random script

   session_start();

   if (empty($_SESSION['rand_code']))
   {
       $str = "";
       $length = 0;
       for ($i = 0; $i < 4; $i++)
       {
             // this numbers refer to numbers of the ascii table (small-caps)
             $str .= chr(rand(97, 122));
       }
       $_SESSION['rand_code'] = $str;
   }

   $imgX = 60;
   $imgY = 20;
   $image = imagecreatetruecolor(60, 20);
   $backgr_col = imagecolorallocate($image, 238,239,239);
   $border_col = imagecolorallocate($image, 208,208,208);
   $text_col = imagecolorallocate($image, 46,60,31);
   imagefilledrectangle($image, 0, 0, 60, 20, $backgr_col);
   imagerectangle($image, 0, 0, 59, 19, $border_col);
   $font = "VeraSe.ttf";

   // it's a Bitstream font check www.gnome.org for more
   $font_size = 10;
   $angle = 0;
   $box = imagettfbbox($font_size, $angle, $font, $_SESSION['rand_code']);
   $x = (int)($imgX - $box[4]) / 2;
   $y = (int)($imgY - $box[5]) / 2;
   imagettftext($image, $font_size, $angle, $x, $y, $text_col, $font, $_SESSION['rand_code']);
   header("Content-type: image/png");
   imagepng($image);
   imagedestroy ($image);
?>

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