PHP Class STATIC for PDO MySQL

class database {

    static private $pdo;

    static function connect($host, $db, $login, $password) {
        self::$pdo = new PDO ("mysql:host={$host};dbname={$db}", $login, $password);
        if(self::$pdo) {
            self::$pdo->query("SET NAMES 'utf8'");
            return true;
        } else
            return false;

    }

    static function query($query, $params = array(), $arr = null) {
        $res = self::$pdo->prepare($query);
        if($res) {
            $res->execute($params);
            return $res->fetchAll($arr ? PDO::FETCH_NUM : PDO::FETCH_ASSOC);
        } else
            return false;
    }

    static function query_single_col($query, $params = array()) {
        $res = self::$pdo->prepare($query);
        if($res) {
            $res->execute($params);
            $val = $res->fetch(PDO::FETCH_NUM);
            return $val[0];
        } else
            return false;
    }

    static function query_single_row($query, $params = array(), $arr = null) {
        $res = self::$pdo->prepare($query);
        if($res) {
            $res->execute($params);
            $val = $res->fetch($arr ? PDO::FETCH_NUM : PDO::FETCH_ASSOC);
            return $val;
        } else
            return false;
    }

}