Also I don’t really understand why the PHP causes JS troubles.
Do you suspect any code-part?
There is no “JS trouble”. The frontend does not change with PHP 8.2, the backend does. And if the backend fails to add the neccessary data-attributes to the image links which the frontend needs, then the images will not open in a lightbox.
Lightbox with Photoswipe also works fine with PHP 8.2. However since PHP 8.1 introduced some breaking changes it is very likely that some other plugin or your theme may not be ready for PHP 8.1 or 8.2 yet and causes some problem. Please check the logs of your web server for any error messages and also check if all plugins and your theme officially supports PHP 8.1 or newer.
Since PHP 8.0 still gets security updates until November 2023 (also see , you may also just upgrade to that version. It’s very likely that this may work better.