adodb makes a good DB abstraction layer.
http://adodb.sourceforge.net/
PHP Code:
include('/path/to/adodb.inc.php');
$DB = NewADOConnection('mysql');
$DB->Connect($server, $user, $pwd, $db);
Then if you're working with multiple databases, just make an array or object of NewADOConnection members. It might be wise to make $DB global as well.
PHP Code:
include('/path/to/adodb.inc.php');
global $DB;
$DB = new stdClass;
$DB->foo = NewADOConnection('mysql');
$DB->bar = NewADOConnection('mysql');
$DB->foo->Connect($server, $user, $pwd, $db);
$DB->bar->Connect($server, $user, $pwd, $someotherdb);