<?php
 
 $gelen=$_SERVER['HTTP_REFERER'];
 $sitex="http://www.sitex.comm"; 
 $sitey="http://www.sitey.comm";
 //kodla
 
 if($gelen==$sitex)
 { 
require_once "a.php";
}
 
 elseif($gelen==$sitey) 
{
require_once "b.php";
}
 else
 {
require_once "default.php";
}
 
 ?>