Using PHP files from Ajax and from other PHP files

I want move all database access into separate PHP files so I could use these files either through an Ajax call from the client or in another PHP files (require_once) on the server. Is this possible? What interface would be needed? Has anybody else tried something similar? Since I plan use Json as the data format for Ajax what overhead would that mean to decode again already Json encoded result if used on the server? Is there a workaround for Json in case of Ajax but none when using in other PHP files? O. Wyss