multi-db.php

<?php $URL = explode('.', $_SERVER[HTTP_HOST]);

if($URL[0] == 'dev') {
    define('DB_NAME', 'dbname_dev');
    define('DB_USER', 'dbuser');
    define('DB_PASSWORD', 'dbpass');
    define('DB_HOST', 'dbhost');
} else {
    define('DB_NAME', 'dbname');
    define('DB_USER', 'dbuser');
    define('DB_PASSWORD', 'dbpass');
    define('DB_HOST', 'dbhost');
}