Interpretation of URL-parts

URL containing php-Redirect where there should be a directory

[nluetic]

Hello,

the following URL:
[site.com...]

contains redir.php, which is of course a file, not a directory. The script defines a PHP-Session and does a redirect to some other directory (newdir). This new directory contains the rest of the path ( search/test.php).
Calling this URL results in:
[site.com...]

Why is this URL interpreted the way it is (btw: only under Linux/Apache, not Windows/Apache or Windows/IIS)?

Greetings

Neven