Best way to create a preference-file?

I would like to create a file that holds the preferences for a site, for example the name of the database, the size of thumbnail pictures etc… But which is the best way to do this and to access the file? It would be nice with an xml-file, but perhaps it is an overkill… Anyway it must be possible to update the file in an easy way, that is, not rewrite it but update it… Sincerely Victor