老师想问一下为什么我这个代码显示不出图片呢？ 谢谢

<?php

require_once 'string.func.php';

$width=80;

$height=28;

$image=imagecreatetruecolor($width, $height);

$white=imagecolorallocate($image, 255, 255, 255);

$black=imagecolorallocate($image, 0, 0, 0);

imagefilledrectangle($image, 1, 1, $width-2,$height-2, $black);

//imagefilledrectangle($image, 1, 1, $width-2,$height-2, $white);

$type=1;

$length=4;

$chars=buildRandomString($type,$length);

$sess_name="verify";

$_SESSION[$sess_name]=$chars;

$fontfiles=array("ALGER.TTF","ARLRDBD.TTF","BASKVILL.TTF","BERNHC.TTF","ITCBLKAD.TTF");

for ($i=0; $i<$length; $i++) {

$size=mt_rand(14, 18);

$angel=mt_rand(-15, 15);

$x=5+$i*$size;

$y=mt_rand(20, 26); 

$fontfile="../fonts/".$fontfiles[mt_rand(0, count($fontfiles)-1)];

$color=imagecolorallocate($image, mt_rand(50, 90),mt_rand(80, 200),mt_rand(90, 180));

$text=substr($chars, $i,1);

imagettftext($image, $size, $angle, $x, $y, $color, $fontfile, $text);

}

header("content-type:image/gif");

imagegif($image);

imagedestroy($image);

?>