Setting Environment Variables

Setting environment variables in your server configuration
on a host-by-host basis allows you to configure virtual hosts differently.

<?php
putenv('ORACLE_SID=ORACLE'); // configure oci extension
?>

Adjusting behavior based on an environment variable

 <?php
$version = $_SERVER['SITE_VERSION'];

// redirect to http://guest.example.com, 

//if user fails to sign in correctly
if ('members' == $version) {
    if (!authenticate_user($_POST['username'], $_POST['password'])) {
        header('Location: http://guest.example.com/');
        exit;
    }
}
include_once "${version}_header"; // load custom header

http://www.php.net/putenv; information on setting environment 
variables in Apache at