报错Warning: mysqli::set_charset(): Couldn't fetch mysqli in

<?php

$mysqli=@new mysqli('localhost','root','root','secondclass');

$mysqli->set_charset('utf-8');

$sql=<<<EQF

CREATE TABLE IF NOT EXISTS mysqli(

id TINYINT UNSIGNED AUTO_INCREMENT KEY,

username VARCHAR(45) NOT SQLITE3_NULL

);

EQF;

$res=$mysqli->query($sql);

var_dump($res);

?>