在php上创建验证码图像

我有一个代码来创建cpatcha图像上的飞行，它显示图像为破碎…下面是代码

 $code = $this->generateCode($characters);
  /* font size will be 95% of the image height */
  $font_size = $height * 1;
  $image = imagecreate($width, $height) or die('Cannot initialize new GD       image stream');
  /* set the colours */
  $background_color = imagecolorallocate($image, 255, 255, 255);
  $text_color = imagecolorallocate($image,0, 0, 0);
  //      $text_color_2 = imagecolorallocate($image, 100, 40, 20);
  $noise_color = imagecolorallocate($image, 50, 40, 35);
  $line_color = imagecolorallocate($image, 150, 140, 135);
   //      $noise_color_2 = imagecolorallocate($image, 150, 120, 100);
  /* generate random dots in background */
  for( $i=0; $i<($width*$height)/15; $i++ ) {
     imagefilledellipse($image, mt_rand(0,$width), mt_rand(0,$height), 1, 1,  $noise_color);
  }
  /* generate random lines in background */
  for( $i=0; $i<($width*$height)/150; $i++ ) {
     imageline($image, mt_rand(0,$width), mt_rand(0,$height),     mt_rand(0,$width), mt_rand(0,$height), $line_color);
  }
  /* create textbox and add text */
  $textbox = imagettfbbox($font_size, 0, $this->font, $code) or die('Error in imagettfbbox function');
  $x = ($width - $textbox[4])/2;
  $y = ($height - $textbox[5])/2;
  imagettftext($image, $font_size, 0, $x, $y, $text_color, $this->font , $code) or die('Error in imagettftext function');
  /* output captcha image to browser */
  header('Content-Type: image/jpeg');
  imagejpeg($image);
  imagedestroy($image);

谁能说我的代码有什么问题?