关于使用PDO连接数据库的一些基本操作

<?phpheader("content-type:text/html;charset=utf-8");$dsn="mysql:dbname=EC_ERP_v2;host=127.0.0.1";$db_user='WebDev';$db_pass='WebDev';try{    $pdo=new PDO($dsn,$db_user,$db_pass);}catch(PDOException $e){    echo '数据库连接失败'.$e->getMessage();}$openid = '94782645F82DCACFa';$sql="select * from erp_user where user_qq='$openid'; ";$rmt=$pdo->query($sql);$res = $rmt->fetchAll();if(count($res)>0){    echo '用户存在';    echo '<pre>';    var_dump($res);    echo '</pre>';}else{    $username = 'admin';    $userpsw = md5('123456');    $userheadurl ='http://q.qlogo.cn/qqapp/1404053/94782645F82DCACF41BF9246AEF1';    $time = time();    $sqla = "insert into erp_user (user_name,user_pwd,user_headurl,user_qq,user_createtime) values ('$username','$userpsw','$userheadurl','$openid','$time');";    $rest = $pdo->exec($sqla);    $aa =$pdo->errorInfo();//可以打印错误信息；    var_dump($aa);    echo '影响行数：'.$rest;}//新增$sql="insert into buyer (username,password,email) values ('ff','123456','[e-mail]')";$res=$pdo->exec($sql);echo '影响行数：'.$res;//修改$sql="update buyer set username='ff123' where id>3";$res=$pdo->exec($sql);echo '影响行数：'.$res;//查询$sql="select * from buyer";$res=$pdo->query($sql);foreach($res as $row){    echo $row['username'].'<br/>';}//删除$sql="delete from buyer where id>5";$res=$pdo->exec($sql);echo '影响行数：'.$res;?>