<?php

$dbCredentials = array(
    'id1' => array(
        'host' => 'host1',
        'user' => 'user1',
        'pass' => 'pass1',
        'name' => 'name1',
    ),
    'id2' => array(
        'host' => 'host2',
        'user' => 'user2',
        'pass' => 'pass2',
        'name' => 'name2',
    ),
);

if (!array_key_exists($db_selection, $dbCredentials)) {
    echo 'Error!';
    die;
}

$conn = mysql_connect($dbCredentials[$db_selection]['host'], $dbCredentials[$db_selection]['user'], $dbCredentials[$db_selection]['pass']); 
mysql_select_db($dbCredentials[$db_selection]['name']); 

Error!