when using Apache you might consider a apache_lookup_uri on the path, to discover the real path, regardless of any directory trickery.
then, look at the prefix, and compare with a list of allowed prefixes.
for example, my source.php for my website includes:
if(isset($doc)) {
$apacheres = apache_lookup_uri($doc);
$really = realpath($apacheres->filename);
if(substr($really, 0, strlen($DOCUMENT_ROOT)) == $DOCUMENT_ROOT) {
if(is_file($really)) {
show_source($really);
}
}
}
hope this helps
regards,
KAT44