Php Captcha没有显示图像

Question

我在我的联系表单上安装了captcha.php但由于某种原因它目前没有显示图像。     

$backgr_col = imagecolorallocate($image, 238,239,239);
$border_col = imagecolorallocate($image, 208,208,208);
$text_col = imagecolorallocate($image, 46,60,31);

imagefilledrectangle($image, 0, 0, 100, 37, $backgr_col);
imagerectangle($image, 0, 0, 99, 36, $border_col);

$font = "bell-centennial-address-bt.ttf"; // it's a Bitstream font check <a rel="nofollow" href="http://www.gnome.org" rel="nofollow">http://www.gnome.org</a> for more
$font_size = 12;
$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);