I have set up a test web server with ISPConfig to try a PHP shopping system (osCommerce). PHP scripts work but when I try to install the shopping cart system it does nothing at step 2. In the Apache error log it just has this:
[error] an unknown filter was not added: PHP

Does anyone have any suggestions for troubleshooting this problem?

I had this issue myself until I found the following info about httpd.conf directives:

Since 4.3.2RC the default Apache 2 sapi is the apache2handler and is no
longer filter based. Therefor you should use
AddType application/x-httpd-php .php
AddType application/x-httpd-php-source .phps

and not

<Files *.php>
SetOutputFilter PHP
SetInputFilter PHP
</Files>

If you do want to use the old filter sapi, you need to use
--with-apxs2filter option.

...I had BOTH listed in my httpd.conf, commenting out the Filter directives solved the issue.

- Andy

I am getting this same error when I try to access the file system, read or write a file. I tripled checked permissions, they seem OK. I have the same PHP code running on Fedora 4, but I cannot get it to run on SUSE 10.0. I really believe it is a configuration problem, but do not know where?

Anyhelp would be much appreciated.