<?php
$Refereal_ID = 2;
switch($Refereal_ID) {
case 2:
$email_from = 'email2@ya.ru';
break;
case 3:
$email_from = 'email3@ya.ru';
break;
default:
$email_from = 'email999@ya.ru';
break;
}
// ...
$headers = "From: $email_from \r\n";
var_dump($headers);