As said by mfv30, there is no "PHP or Apache or Lighttpd problem" to solve, because he says that all php scripts were working fine but install.php not.
I had the same problem here, and found the solution somewhere on the web (can't remember where)...
If you are trying to access install.php with the uri : "http://localhost/dokuwiki/install.php
", it will simply fail and the web browser will try to download the file (tested from dokuwiki version 2008 to the latest !!)
You HAVE TO use a different uri to run the "install.php" script, like : "http://127.0.0.1/dokuwiki/install.php
This should be declared as a bug, but i do not have the time to...