create a file called 'dbtest.php' in your root osticket folder.
Put this in it:
<?php
DEFINE ('DB_USER', 'username');
DEFINE ('DB_PASSWORD', 'password');
DEFINE ('DB_HOST', 'IP address of DB server');
DEFINE ('DB_NAME', 'database name');
$dbc = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
if(!$dbc){
die('error connecting to database: ' . mysqli_connect_error()););
}
?>
update that script to use the username, password, ip address of the mysql server and database name.
Then browser to your site\dbtest.php
What does it tell you?