Connection database php5.5

Question

I want to connect to my database using pdo , but in php version 5.5 and php7 not work. This is my code,

 public static function getInstance() {

    if (!isset(self::$instance)) {
        try {
        //    $charset = array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8");
            self::$instance = new PDO(DNS, USER, PASSWORD, array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));
           // self::$instance->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_WARNING);
            echo'connexion reussie';
        } catch (Exception $e) {
            echo "failed to connect : ", $e->getMessage();
            die();
        }
    }

    return self::$instance;
}

how I can modify this code to be working with php 5.5 and php 7.

Thanks for helps