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
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;
?>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<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>
</HEAD>
<BODY>
<form name="loginfrom" action="php/userController.php?flag=login"method="POST">
<DIV id=div1>
  <TABLE id=login height="100%" cellSpacing=0 cellPadding=0 width=800 
align=center>
    <TBODY>
      <TR id=main>
        <TD>
          <TABLE height="100%" cellSpacing=0 cellPadding=0 width="100%">
            <TBODY>
 
              <TR>
                <TD colSpan=5>&nbsp;</TD>
           
              <TR height=30>
                <TD width=380>&nbsp;</TD>
<TD>&nbsp;</TD>
                <TD>&nbsp;</TD>
                <TD>&nbsp;</TD>
                <TD>&nbsp;</TD>
              </TR>   
</TR>
<TR height=20>
<TD width>&nbsp;</TD>
<TD><font size="3"><b>登录<b></font></TD>
<td>
<input type="radio" name="account" value="student"checked>学生
<input type="radio" name="account" value="teacher">教师
<td></TR>
</TR><TR height=20>
              <TR height=40>
                <TD rowSpan=4>&nbsp;</TD>
                <TD>用户名：</TD>
                <TD>
                  <INPUT class=textbox id="user_name" name="user_name">
                </TD>
                <TD width=120>&nbsp;</TD>
              </TR>
              <TR height=40>
                <TD>密　码：</TD>
                <TD><INPUT class=textbox id="user_password" type="user_password"
            name=txtUserPassword>
                </TD>
                <TD width=120>&nbsp;</TD>
              </TR>
              <TR height=40>
                <TD>验证码：</TD>
<TD vAlign=center colSpan=2>
                  <INPUT id="identifying" size="4" name="identifying">
                  &nbsp; <img src="img.php"/> <A id=LinkButton1 href="#">不清楚，再来一张</A></TD>
              </TR>
              <TR height=40>
                <TD></TD>
                <TD align=right>
                  <INPUT id="submit" type="submit" value=" 登 录 " name="submit" onclick="return check()">
                </TD>
                <TD width=120>&nbsp;</TD>
              </TR>
              <TR height=110>
                <TD colSpan=4>&nbsp;</TD>
              </TR>
            </TBODY>
          </TABLE>
        </TD>
      </TR>
      <TR id=root height=104>
        <TD>&nbsp;</TD>
      </TR>
    </TBODY>
  </TABLE>
</DIV>
<DIV id=div2 style="DISPLAY: none"></DIV>
</CONTENTTEMPLATE>
</form>
</BODY>
</HTML>