The following global variables are accessible from within files included by the phpinclude skin/templatevar:

$GATEWAY_INTERFACE, $SERVER_NAME, $SERVER_SOFTWARE
$SERVER_PROTOCOL, $REQUEST_METHOD, $QUERY_STRING
$DOCUMENT_ROOT, $HTTP_ACCEPT, $HTTP_ACCEPT_CHARSET
$HTTP_ACCEPT_ENCODING, $HTTP_ACCEPT_LANGUAGE
$HTTP_CONNECTION, $HTTP_HOST, $HTTP_REFERER
$HTTP_USER_AGENT, $REMOTE_ADDR, $REMOTE_PORT
$SCRIPT_FILENAME, $SERVER_ADMIN, $SERVER_PORT
$SERVER_SIGNATURE, $PATH_TRANSLATED, $SCRIPT_NAME
$REQUEST_URI, $argv, $argc, $PHP_SELF
$HTTP_COOKIE_VARS, $HTTP_GET_VARS, $HTTP_POST_VARS
$HTTP_POST_FILES, $HTTP_ENV_VARS, $HTTP_SERVER_VARS
$HTTP_SESSION_VARS, $PATH_INFO, $HTTPS
$HTTP_RAW_POST_DATA, $HTTP_X_FORWARDED_FOR

For others variables, you'll need to add 'global $varname;' explicitly in your code