如何执行mysqli不能执行update语句？

$update_query = “UPDATE users SET credits = (credits – ‘$cred’) WHERE user_id = (‘$u’)”;

此语句用mysqli无法执行，无报错。后改用原生的mysql解决

$con = mysql_connect(“localhost”,”peter”,”abc123″);

mysql_select_db(“my_db”, $con);

mysql_query($update_query);