The dirname function does not usually return a slash on the end, which might encourage you to create links using code like this:
$url = dirname($_SERVER['PHP_SELF']) . '/somepage.php';
However dirname returns a slash if the path you specify is the root, so $url in that case would become '//somepage.php'. If you put that URL as the action on a form, for example, submitting the form will try to go to http://somepage.php.
I ran into this when I wrote a site on a url with a path, www.somehost.com/client/somepage.php, where the code above works great, but then wanted to put it on a subdomain, client.somehost.com/somepage.php, where things started breaking.
The best solution would be to create a function that generates absolute URLs and use that throughout the site, but creating a safe_dirname function (and an htaccess rewrite to fix double-slashes just in case) fixed the issue for me:
<?php
function safe_dirname($path)
{
$dirname = dirname($path);
return $dirname == '/' ? '' : $dirname;
}
?>