PHP is great in a sense that you can easily combine legacy and modern code in the same codebase. Just do a new_index.php for all new stuff. I'd start building n

PHP is great in a sense that you can easily combine legacy and modern code in the same codebase. Just do a new_index.php for all new stuff. I'd start building new features and features that are in active development in the 'modern way', and just keep the legacy code as is. When the new way has been established, and the team has accustomed to it, it becomes easier gradually rewrite old parts, when necessary. You might find out that lots of the old code doesn't need to be rewritten, but can be managed as a legacy part of the app which means mostly frozen but working code. You should also understand the audience. Who are the users of the app? It sounds like that the app does not need high reliability or availability, or any of the stuff that's required for typical mass market web apps. Understanding this might give you some room to improvise.