为什么用这段代码登记会无法显示

$conn=mysql_connect('localhost','root','')or die('数据库连接错误');
mysql_select_db('demo',$conn);
mysql_query("set names 'gbk'");
//检测会员名称是否存在
if($_post[username]){
  $sql="select * from users where username='$_post[username]'";
  $res=mysql_query($sql)or die("查询失败！");
  $us=$_get[name];
  echo $us."222222";
  $ps教程= md5($_post[password]);
  $sex=$_post['sex'];
  $age=$_post['age'];
  $qq=$_post['qq'];
  $phone=$_post['phone'];
  $email=$_post['email'];
  $address=$_post['address'];
  if(is_array(mysql_fetch_row($res))){
       echo "该会员名已经存在！";
  }else{
        $sql="insert into users (username,password,sex,age,qq,phone,email,address) values ('$us','$ps',$sex,$age,$qq,'$phone','$email','$address')";
        echo $sql;
       mysql_query($sql)or die("注册失败");
       echo "";
  }
}
?>