That depends; if you have more than two values, it becomes much easier to do something like:
$vals = array("ZA", "GB", "US", "etc");
if(in_array($country, $vals))) {
// do this - country is in valid list of responses
} else {
// not valid
}
EDIT: Misread your question; for two elements you can combine the two conditionals:
if($country == "ZA" || $country == "GB") {