Hází chybu php Captcha

<?php
session_start();
class CaptchaSecurityImages {

   var $font = 'arial.ttf';

   function generateCode($characters) {
      /* list all possible characters, similar looking characters and vowels have been removed */
      $possible = 'abcdefghijklmnopqrstuvwxyz';
      $code = '';
      $i = 0;
      while ($i < $characters) {
         $code .= substr($possible, mt_rand(0, strlen($possible)-1), 1);
         $i++;
      }
      return $code;
   }
////////////////////Thanks for http://www.white-hat-web-design.co.uk/////////////////////////////
   function CaptchaSecurityImages($width='70',$height='20',$characters='5') {
      $code = $this->generateCode($characters);
      /* font size will be 60% of the image height */
      $font_size = $height * 0.6;
      $image = imagecreate($width, $height);
      /* set the colours */
      $background_color = imagecolorallocate($image, 255, 255, 255);
      $text_color = imagecolorallocate($image, 20, 40, 100);
      $noise_color = imagecolorallocate($image, 100, 120, 180);

      /* create textbox and add text */
      $textbox = imagettfbbox($font_size, 0, $this->font, $code);
      $x = ($width - $textbox[4])/2;
      $y = ($height - $textbox[5])/2;
      imagettftext($image, $font_size, 0, $x, $y, $text_color, $this->font , $code);
      /* output captcha image to browser */
      header('Content-Type: image/jpeg');
      imagejpeg($image);
      imagedestroy($image);
      $_SESSION['vcode'] = $code;
   }

}

$width = isset($_GET['width']) && $_GET['width'] < 600 ? $_GET['width'] : '70';
$height = isset($_GET['height']) && $_GET['height'] < 200 ? $_GET['height'] : '20';
$characters = isset($_GET['characters']) && $_GET['characters'] > 4 ? $_GET['characters'] : '5';
$captcha = new CaptchaSecurityImages($width,$height,$characters);
?>

Fatal error: Call to undefined function: imagecreate() in /home/ftponly/usr/fwitpekc/web/comments/captcha.php on line 23