这个php登陆代码总是不成功。请看看这是哪里有问题 ？
<?php<br />
<br />
session_start();<br />
$conn=mysql_connect("localhost","admin","admin") or die ("数据库链接错误".mysql_error());<br />
mysql_select_db('member',$conn) or die("选择数据库有错".mysql_error());<br />
<br />
define(ALL_PS, "php123");<br />
<br />
if($_POST[submit])<br />
{<br />
$username=str_replace(" ","",$_POST[username]);<br />
$sql="select * from user_list where 'username' ='$username'";<br />
$query=mysql_query($sql);<br />
$ps = $us ? md5($_POST[password].ALL_PS)==$row[password] : FALSE;<br />
<br />
if($ps){<br />
$_SESSION[uid]=$row[uid];<br />
$_SESSION[user_shell]=md5($row[username].$row[password].ALL_PS);<br />
echo "ok";<br />
}else{<br />
echo "no";<br />
}<br />
<br />
}<br />
<br />
?><br />
<br />
<form action="" method="post""><br />
用户：<input type="text" name="user" /><br /><br />
密码:<input type="password" name="password" /><br /><br />
<input type="submit" name="submit" value="tttt" /><br />
</form><br />
if($_POST['submit'])<br />
{<br />
$username=str_replace(" ","",$_POST['username']);<br />
$sql="select * from user_list where `username` ='$username'";<br />
$query=mysql_query($sql);<br />
$row = mysql_fetch_assoc($query);<br />
$ps = $row['username'] ? md5($_POST['password'].ALL_PS)==$row[password] : FALSE;<br />
