<?php
$gelen = $_SERVER['HTTP_REFERER'];
$sitex = "sitex.com";
$sitey = "sitey.com";

//kodla
if(strpos($gelen, $sitex)){
include('a.php');

}elseif(strpos($gelen, $sitey)){
include('b.php');

}else{
include('default.php');
}
?>