The manual say for realpath():

"[...]The resulting path will have no symbolic link, '/./' or '/../' components."

This is not true for '.' and '..':

[...]
D:\dev\php\http\tests>php
<?
print realpath('./');
^Z
X-Powered-By: PHP/4.0.1pl2
Content-type: text/html

D:\dev\php\http\tests\./
[...]

using '..' even breaks the path:

[...]
D:\dev\php\http\tests>php
<?
print realpath("..");
^Z
X-Powered-By: PHP/4.0.1pl2
Content-type: text/html

D:\dev\php\http./
[...]