GWT can be used with PHP the same as jQuery, Angular, React, Babel, TypeScript, etc. can be used with PHP.
You won't be able to use GWT-RPC or RequestFactory to communicate with your server, but most people have moved away from these for new projects anyway: you can use GWT's RequestBuilder, raw XMLHttpRequest, or
fetch() to make HTTP requests to your server where the PHP code runs, with JsInterop to map JSON responses to Java objects; or you could go with things like
domino-rest.
For running your code during development, you'll want to use the CodeServer with -launcherDir pointing to your Apache DocumentRoot (or whatever equivalent tooling you're using to serve your PHP code), or DevMode in -noserver mode (with -war pointing to the DocumentRoot in this case)
I'm afraid I don't have tutorials though, even less ones that would be up-to-date with GWT tooling and state-of-the-art…