[thijsvandien]

The infamous rewrites that Joel Spolsky once urged to never do, are those where development on the current version is halted while a brand new version is developed from scratch and then launched with a big bang. Without answering the question directly, web applications are some of the easiest to rewrite gradually, endpoint by endpoint. Especially in PHP, where requests are stateless. You could start a new thing in the middle of the old thing and slowly let it take over, while already live and working in tandem with the old parts.