php验证码有关问题

php验证码问题
linux下php5 安装了php5-gd，但是验证码图片还是不能显示求解为何？


图片显示为这样




img.php

PHP code

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<?php session_start();
$width=50;
$height=20;
header("Content-type:image/gif");
$randval=randStr(4);
$im=imagecreatetruecolor($width,$height);
$r=array(255,215,235,195,175);
$g=array(255,215,235,195,175);
$b=array(255,215,235,195,175);
$key=rand(0,4);
$backColor=imagecolorallocate($im,$r[$key],$g[$key],$b[$key]);
$pointColor=imagecolorallocate($im,255,170,255);
imagefill($im,0,0,$backColor);
$stringColor=imagecolorallocate($im,0,0,0);
for($i=0;$i<=100;$i++){
    $pointX=rand(2,$width-2);
    $pointY=rand(2,$height-2);
    imagesetpixel($im,$pointX,$pointY,$pointColor);
}
imagestring($im,3,5,1,$randval,$stringColor);
$ImageFun='Imagegif';
$ImageFun($im);
imagedestroy($im);
function randStr($len=10){
    $chars='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    $string="";
    while(strlen($string)<$len){
        $string.=substr($char,(rand()%strlen($chars)),1);
    }
    return $string;
}
$_SESSION['identifying']=$randval;
?>

PHP code

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?


<title>登录</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="images/public.css" type="text/css" rel="stylesheet">
<link href="images/login.css" type="text/css" rel="stylesheet">
<style type="text/css">
</style>
<meta content="MSHTML 6.00.2900.5848" name="GENERATOR">
<script language="javascript">
function check(){
    if(docnument.getElementByID("txtUserName").value==''){
        alert("用户名不允许为空");
        docnument.getElementById("txtUserName").focus();
        return false;
    }
    if(docnument.getElementById("txtUserPassword").value==''){
        alert("密码不能为空");
        docnument.getElementById("txtUserPassword").focus();
        return false;
    }
    return true;
}
</script>


    

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