Based on his popular conference session "It was like that when I got there", Paul Jones has announced the release of an ebook covering the migration of legacy applications to a more recent, well-structured system. The book, "Modernizing Legacy Applications in PHP", is still in the works, though.
My upcoming book, "Modernizing Legacy Applications in PHP", [shows how to improve your application with small changes]. Using my talk "It Was Like That When I Got Here" as a starting point, I condense 15 years of fixing PHP codebases into a collection of specific steps to complete in order. As you apply these small fixes, each one building on the last, you will be able to restructure your codebase from a spaghetti mess to a testable, organized, modernized application, free of globals and mixed concerns.
If the premise sounds interesting, be sure to sign up to be notified of updates as they happen. There's also some freebies that'll come along with the subscription including a pre-publication chapter.