<?php
header('content-type:text/html;charset=utf-8');
$mysqli=new?mysqli('localhost','root','','tamia');
if?($mysqli->errno)?{
????die("Connect?Error:".$mysqli->error);
}
$mysqli->set_charset('UTF8');
$username=$_POST['username'];
$password=md5($_POST['password']);

/*$sql="SELECT?*?FROM?users?WHERE?username='{$username}'?AND?password='{$password}'";
$mysqli_result=$mysqli->query($sql);
if($mysqli_result?&&?$mysqli_result->num_rows>0){
????echo?'登錄成功';
}else{
????echo?'登錄失敗';
}*/

$sql="SELECT?*?FROM?user?WHERE?username=??AND?password=?";
$mysqli_stmt=$mysqli->prepare($sql);
$mysqli_stmt->bind_param('ss',$username,$password);
if?($mysqli_stmt->execute())?{
????$mysqli_stmt->store_result();
????if?($mysqli_stmt->num_rows>0)?{
????????echo?'登錄成功';
????}else{
????????echo?'登錄失敗';
????}
}
//釋放結果集
$mysqli_stmt->free_result;
//關閉預處理語句
$mysqli_stmt->close();
//關閉連接
$mysqli->close();

?>

老師的源代碼也是如此，請問是什么問題