    public static function exist($tag) {

        $sql = "SELECT `name` FROM `cat` WHERE `name` = '{$tag}'";
        $q = Db::result($sql);
        if (Db::$num_rows > 0) {
            return true;
        }else{
            return false;
        }
    }

The software constructs all or part of an SQL command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended SQL command when it is sent to a downstream component.