Linux – Can’t connect to MariaDB

I've a problem that I can't figure it out by myself.

I'm trying to host a website on my VPS server:

-Debian 9

-MariaDB

-PHP 7.0.19-1

My website is attempting to connect to my database (which is accessible through phpmyadmin) but with no success.

PHP code:

<?php
$cfg['db_server'] = 'ipaddress'; 
$cfg['db_user'] = 'user';
$cfg['db_pass'] = 'password'; 
$cfg['db_name'] = 'newdb';

$conn = @mysqli_connect ($cfg['db_server'], $cfg['db_user'], $cfg['db_pass']);
$select = @mysqli_select_db ($cfg['db_name'], $conn);

if (!$conn) {
    die ('<p class="error">Can't connect to server</p>');
}
if (!$select) {
    die ('<p class="error">Can't select db.</p>');
}
?>

On my website I see "Can't connect to server" error.

Could you kindly help me to troubleshoot this problem? I don't have much experience with linux systems, so I don't how to find a solution for this. If you need more information just, I will send it ASAP.

Thank you in advance