Success or Failed message not displaying after PHP script execution for database connection

Question

This is constant.php script. As usual I am getting a "Successful or Failed" message for connection. Also it is not outputting any error message. please do tell in the comments).

<?php
define("HOST","localhost");
define("USER","root");
define("PASS","");
define("DB","project_inv");
define("DOMAIN","http://localhost/inv_project/public_html");
?>

This db.php script,after executing script it doesn't show success or failed message.

class Database
{

    private $conn;

    public function connect()
    {
        include_once("constants.php");
        $this->conn = new mysqli(HOST,USER,PASS,DB);

        if ($this->conn) {

         echo "connected";
         // return $this->conn;
        }else{

            return "Database Connection Faild";
        }


    }
}