php连接数据库的几种方式

<?php
$name = $_POST["u_name"]; //flag1:$_POST["u_name"]
$pass = $_POST["u_pass"];
$sql2 = "select * from admin where username = ? and password= ?";
$db = new PDO('mysql:host=127.0.0.1;dbname=login;charset=UTF-8', 'root', '123456789');
//flag2:new PDO('mysql:host=127.0.0.1;dbname=login;charset=UTF-8','root', '123456789');
$sth = $db->prepare($sql2); //flag3:$db->prepare($sql2);
$sth->bindParam(1,$name,PDO::PARAM_STR,12);
//flag4:$sth->bindParam(1,$name,PDO::PARAM_STR,12);
$sth->bindParam(2,$pass,PDO::PARAM_STR,12);
$sth->execute();
$res = $sth->fetch();
//flag5:$res = $sth->fetch();
if (empty($res))
{
echo "错误！";
// print_r($sth->errorInfo());
}else{
echo "Success!!!!";
}