关于PHP会员登录验证码问题？

<?php
/*
 Admin登陆
 */
include("conn.php");
session_start();
if($_POST[cehck]){
     if($_POST[cehck]==$_SESSION[check_pc]){
md5("admin".ALL_PS);
if($_POST[submit]){
$username=$_POST['username'];
$sql="select * from user_list where username='$username'";
$query=mysql_query($sql);
$us=is_array($row=mysql_fetch_array($query));
$ps=$us ? md5($_POST[password].ALL_PS)==$row[password] : FALSE;
if($ps){
    $_SESSION[uid]=$row[uid];
    $_SESSION[user_shell]=md5($row[username].$row[password].ALL_PS);
    $_SESSION[times]=mktime();//登陆的时间
    echo "<center><b><font color=red>登陆成功</font></b>";
}else{
    echo "验证码及用户密码错误";
}

}
     }
}

?>

<h1></h1>
<h2 align="center">管理员登陆</h2>
<center><form action="" method="post" name="myformn" onsubmit="return chekpost()">
管理员:<input type="text" name="username"/><br/>
密&nbsp;&nbsp;码:<input type="password"size="21" name="password"/><br>
验证码: <input type="text" size="10" name="cehck"/><img src="che.php"><br>
<input type="submit" name="submit" value="提交"/>
<input type="reset" name="rest" value="重置"/
  </form>