Just installed LAMP following the instructions, I uploaded my code which was working perfecty locally and I got an Error500. Checked the apache logs and getting:

[Tue Apr 17 16:26:54.072762 2018] [:error] [pid 15636] [client 181.14.122.235:50286] PHP Warning:  mysqli_connect(): (HY000/2002): Connection refused in /var/www/mywebsite.com/public_html/dbconnection.php on line 7, referer: http:/mywebsite.com/

My dbconnection file is:

<?php
//Establishing connection with the database
define('DB_SERVER', 'localhost');
define('DB_USERNAME', 'root');
define('DB_PASSWORD', 'mypassword');
define('DB_DATABASE', 'mydbname'); //where customers is the database
$db = mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);
?>

Data is correct since I used it to access phpmyadmin.

What else could this be?
Thanks!