undefind

<?Php
$mysqli = new mysqli("localhost","root","QGoWklyMJlaNxHz7","mtst");

if($mysqli)
{
echo "成功";
$re = $mysqli->query("select * from htst");
echo "select * from htst"." ".$mysqli->num_rows;
var_dump($re);
print_r($re);
while($jgj = $mysqli->fetch_assoc($re))
echo $jgj["hI"]." ".$jgj["hN"]." ".$jgj["hP"]."<p>";
}
else
exit();

?>

请问 为什么 $mysqli->fetch_assoc() 是 未定义方法呢