数据库连接的PHP脚本执行后未显示成功或失败消息

这是constant.php脚本。像往常一样，我收到一条“成功或失败”的连接消息。它也不会输出任何错误消息。请务必在评论中说明）

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

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";
        }


    }
}