我收到错误消息“致命错误：未捕获错误：未定义的类常量'ERRMODE_EXPECTION'

Question

$db_user = "root"; $db_pass = ""; $db_name = "useraccounts";
$db = new PDO('mysql:host=localhost;dbname=' . $db_name . ';charset=utf8', $db_user, $db_pass); 
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXPECTION)