"Show new post since last visit" does not show up

Rickard, thanx for your quick reply.

Step 1
I have checked my PHP environment (PHP version is 4.1.2.) with a file called phpinfo.php (including phpinfo()) and here are the results of the jury:

_SERVER["PHP_SELF"]    /forum/phpinfo.php

Forum is installed in subdirectory /forum/ and the filename seems to be available in the PHP_SELF environment variable.

Step 2
For debugging purposes I have added

echo "PHP_SELF variable is" . $_SERVER['PHP_SELF']

to the header.php and receive the following output:

PHP_SELF variable is /forum/indexphp

The DOT between file name and extension appears to be missing. Is that perhaps of significance?

Questions

- What is the purpose of this IF-statement? Security?

- What happens if I comment the IF (and the else) out?

Does it really output indexphp? Not index.php?

Yes. In the browser window and in the source code.

But it gets even weirder: If I select the string, copied it and on pasting it, it stops after index, even if other text after the ending php was also copied. Like this:

PHP_SELF is /forum/index

Whereas in the source and on the page it definitely reads (I am wearing reading glasses):

PHP_SELF is /forum/indexphp

So there seems to be something invisible between index and php that stops it from being copied. Weird.
Any ideas?

UPDATE:
The above copying was done in Firefox and its source window. In the windows editor (view source in IE) there is a space between index and php and it can be copied:

PHP_SELF is /forum/index php

So there seems to be something in that PHP_SELF variable that probably should not be there.

By "this site" I mean forums.punbb.org
I mean that I get a 404, versus on your site it loads the page

We captured packets with Ethereal and the packets contain the value 0x00 and not 0x2e, which would apparently be a proper DOT. Funnily enough Ethereal shows the wrong value as the right dot.

We currently have no explanation as to why PHP does this. But it at least explains why PHP_SELF fails the IF-statement in header.php and why "Show new post since last visit" in pubBB does not show up.

===
Temporary Solution
I am no programmer, but I figured that the following (unelegant and crude) patch will do. It checks if the strings "index" oder "search" appear in the variable and respectively replace it with the value it ought to have to begin with ("/forum" is just my path). I inserted it in header.php before line 171: 

if (strpos($_SERVER['PHP_SELF'], 'index')) {
  $_SERVER['PHP_SELF'] = "/forum/index.php";  
}    
if (strpos($_SERVER['PHP_SELF'], 'search')) {
  $_SERVER['PHP_SELF'] = "/forum/search.php";  
}

Any suggestions from PHP programmers as to make this statement more elegant are more than welcome.

===
Thank you everybody for your time and concern.