新人关于表单提交问题

form 属性 加了 enctype="text/plain" 取不到 $post[]的值？正常么？
<?php
session_start();
//mysql link
$conn=mysql_connect('localhost','root','');
mysql_select_db('user',$conn);
if($_POST[submit]){
$username= str_replace(" ","",$_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)== $row[password] : FALSE;
echo$ps;
if($ps){
$_SESSION[uid]=$row[uid];
$_SESSION[user_shell]=md5($row[username].$row[password].ALL_PS);
echo "登陆成功";
}else{
echo "密码或者用户名错误";
session_destroy();
}
}else
{
echo $_POST[username];
}
?>
<form action="user.php" method="post" enctype="text/plain">
用户名：
<input type="text" name="username" size="20" maxlength="20"/><br/>
密码：
<input type="password" name="password" size="20" maxlength="20"/><br/>
<input type="submit" name="submit" value="login"/><br/>
</form>