Variables servidor: Host

if ($_SERVER['HTTP_HOST'] == 'wwwserver') { ?>
<base href="http://wwwserver/midominio/"/>
<?php
}
else { ?>
<base href="https://www.midominio.com/"/>
<?php
}

$_SERVER: Variables del servidor

Para que se vaya a la raiz del servidor:
$_SERVER['DOCUMENT_ROOT']

EJ) http://www.uno.com/dos/tres/pepe.php
echo $_SERVER["SERVER_NAME"]; //  www.uno.com
echo $_SERVER["PHP_SELF"]; //  dos/tres/pepe.php
echo $_SERVER["REQUEST_URI"]; //  pepe.php

$IP_REMOTA = $_SERVER["REMOTE_ADDR"];
$IP_PUBLICA = $HTTP_SERVER_VARS["HTTP_X_FORWARDED_FOR"];
$IP_INTERNA = $_SERVER['SERVER_ADDR'];
$nav = $_SERVER['HTTP_USER_AGENT'];  // navegador
$DNS = gethostbyaddr($IP);


---

Detectar si está bajo https:
if (!isset($_SERVER["HTTPS"])) { ?>
<script>location.href = "https://www.midominio.com<?=$_SERVER["PHP_SELF"];?>";</script>
<?php
}