$db[host] indicates your values are coming from an array called $db. Such as...
vars.php
$db = array("hostname, username, password");
If this is the case you need to specify the placeholder or position in the array of the value you want. In the array above the count goes 0, 1, 2. 0 => being hostname, 1 => username, 2 => password.
$connId = mysql_connect($db[0], $db[1], $db[2]);
Or you could just declare the vars outright.
$host = "**";
$user = "";
$pass = "***";