mjakobi wrote:

But what is the right way when I need to make a change to a .php-file?

Depends on your expertise..

The most manageable (if it's more than one patch or a more complicated patch) but not easiest way is to add each release's unzipped files into a Git repository and store individual changes as StGit patches or with git stash; when a new release becomes available add all those files to the repository and then apply the StgGit patches and/or git stash objects. Resolve source code conflicts as they arise and commit the resulting (modified) changes as new StGit patches or git stash objects, resetting HEAD, to be used for the next upgrade.

Even better instead of creating your repository of releases could be to track the original Piwigo Git repository and apply your patches against the release tag's commit chain, which might help git to solve some possible merge conflicts in a 3-way merge, and copy the modified files, and again, resetting HEAD, store the new changes as StGit patches or git stash objects for the next upgrade.

If all that is mumbo-jumbo to you or there's only one or two files to change, make a copy of each to be modified file in its original state, for example

cp -p filename.php filename.php.orig

then modify filename.php and when done produce a patch file with

diff -up filename.php.orig filename.php >filename.php.patch

which after upgrading to a new release can be applied with

patch <filename.php.patch

and again any conflicts arising must be resolved and the resulting diff be stored as a new patch for the next upgrade.