$_SESSION[$sess_name]=$chars;                         $_SESSION[里面是字符哦 ]你少打了单引号     

注释掉 header("content-type:image/gif"); 这个看看

chars=buidRandomString($type,$length);

是不是没有传入$type和$length的值？

截图都看不清代码

<?php
require_once 'string.func.php';
//通过GD库做验证码
//创建画布
$width=80;
$height=20;
$image=imagecreatetruecolor($width, $height);
$white=imagecolorallocate($image, 255, 255, 255);
$black=imagecolorallocate($image,0,0,0);
//用填充矩形填充画布
imagefilledrectangle($image, 1,1,$width-2,$height-2, $white);
$chars=buidRandomString($type,$length);
$sess_name="verify";
$_SESSION[$sess_name]=$chars;
$fontfiles=array("Deng.ttf","Dengb.ttf","Dengl.ttf","msyh.ttc","msyhbd.ttc");
for($i=0;$i<$length;$i++){
	$size=mt_rand(14,18);
	$angle=mt_rand(-15,-15);
	$x=5+$i*$size;
	$y=mt_rand(20,26);
	$color=imagecolorallocate($image,mt_rand(50,90),mt_rand(80,200),mt_rand(90,180));
	$fontfile="../fonts/".$fontfiles[mt_rand(0,count($fontfiles)-1)];
	$text=substr($chars,$i,1);
	imagettftext($image,$size, $angle, $x, $y, $color, $fontfile, $text);
}
header("content-type:image/gif");
imagegif($image);
imagedestroy($image);









<?php
  function buidRandomString($type=3,$length=4)
  {
	  if($type==1)
	  {
		 $chars=join("",range(0,9)); 
	  }
	  else if($type==2)
	  {
		 $chars=join("",array_merge(range("a","z"),range("A","Z")));  
	  }
	  else if($type==3)
	  {
		 $chars=join("",array_merge(range("a","z"),range("A","Z"),range(0,9)));
	  }
	  if($length>strlen($chars))
	  {
		 exit("字符串长度不够");  
	  }
	  $chars=str_shuffle($chars);
	  return substr($chars,0,$length);
  }
?>