Debian major version in place upgrade is usually a very smooth and straightforward process, but there could be some minor glitches where some modules or packages dosen’t follow.
I recently encoutered an issue when upgrading Debian 11 to 12, where the observium page displays the actual PHP code (this happened regardless of the running Observium version, v24 or v26).
This is usually a very good sign that the PHP module isn’t present and/or loaded in Apache.
Follow these steps to troubleshoot and fix the issue :
1. Make sure that PHP itself is present :
php -v
2. Make sure the Apache PHP module is installed, just install if missing :
apt install libapache2-mod-php
3. Enable the module (it should be done automatically at step 2) :
a2enmod php8.2
4. The error should now be different, complaining about missing php-mysqli extension. Just install it :
apt install php-mysqli
5. Observium should now load as expected.