ClearDb Heroku Php 7

I follow this instruction https://devcenter.heroku.com/articles/cleardb#using-cleardb-with-php

to connect my php app(deployed on heroku) to clearDb addons.

class Conn{

    private $server;
    private $database;
    private $username;
    private $password;

    function __construct(){
        $url = parse_url(getenv("CLEARDB_DATABASE_URL"));

        $this->$server = $url["host"];
        $this->$username = $url["user"];
        $this->$password = $url["pass"];
        $this->$database = substr($url["path"], 1);

    }

    function connection(){
        mysqli_report(MYSQLI_REPORT_STRICT);

        try{
            $mysqli = new mysqli($this->server, $this->username, $this->password, $this->database);
        }
        catch (Exception $e ) {
            echo file_get_contents("Pagine/ContattiInizio.xhtml");
            echo "message: " . $e->getMessage();   //This show a "no such a file or directory" error
            exit;
        }

        return $mysqli;
    }

}

new mysqli() throw up an error that I dont really understand "no such a file or directory" and I dont really kwon what to do with that.

EDIT: I tried to connect to my db with tools like dbheaver and it works fine