function curPageURL()
{
    $pageURL = 'http';

    if (isset($_SERVER["HTTPS"])&&($_SERVER["HTTPS"] == "on")) {
        $pageURL .= "s";
    }
    $pageURL .= "://";

    if ($_SERVER["SERVER_PORT"] != "80") {
        $pageURL .= $_SERVER["SERVER_NAME"] . ":" . $_SERVER["SERVER_PORT"] . $_SERVER["REQUEST_URI"];
    }
    else {
        $pageURL .= $_SERVER["SERVER_NAME"] . $_SERVER["REQUEST_URI"];
    }
    return $pageURL;
}

$url=curPageURL();
switch ($url){
    case 'http://a.com':
        include 'index.htm';
        break;
    case 'http://b.com':
        include './b/index.htm';
        break;
    default:
        include 'index.htm';
        break;
}