Server :: Apache Server Not Processing PHP?

I am running the Linux server ( debian etch ) on a virtual machine since more than a year. About a week ago the mail server ( exim4 ) stopped working. I've started aptitude and made something i cannot remember exactly ( said o.k. to an action aptitude proposed me solving some inconsistencies, but aptitude itself crashed several time with "cannot fork" and "SIGSEGV" ) - since then apache2 does not handle the PHP pages, delivers the php source to the browser.

I've tried reinstalling PHP5, but no success so far

- php5 is installed, also the libapache2-mod-php5

- The php5 module is activeted ( the symbolic links stay in mods-enabled. If i remove / rename the /usr/lib/apache2/modules/libphp5.so file, i get an error message when i restart apache2 - it seems me, the PHP5 module is loaded into apache2.

The apache2 configuration file 000-default contains my virtual hosts, among them for example:

<VirtualHost *:80>
ServerName bahnschranke.pxtr.de
DocumentRoot /home/laffin/index_bahnschranke

[code]....

When i try to load a page from this virtual host into my browser, i get the php source code (opened by a texteditor), and no log line is written, neither into error.log, nor into access.log As i said, the system worked correctly for monthes, it is not a new installation, but something went wrong in the last days.