Type hinting only allows for one hint per parameter (and also, the hint needs to be array or a class name, you can't hint string), but you can do this by checking the type of the param within your function, using get_class:
function foo($param)
{
if (!(is_string($param) || in_array(get_class($param), array("Bar", "Baz")))
{
// invalid type for $param!
}
}
You could even use trigger_error to have it fail with a PHP error (like it would if a type hint failed) if you wanted.
