To a certain extent, yes.
The heart of GWT is some magic for converting Java source code for a Web client into JavaScript. There's no wiggle room there; it's either Java or nothing.
But a GWT-translated client can interoperate with a server written in any language. You'd be missing out on some of the special remote calling capabilities offered by the GWT framework, but if you're willing to transfer XML or JSON back and forth, a PHP-based server could work with your Java/JavaScript-based client.
Yes, GWT completely supports PHP. Write your PHP script and use request builder to make the ajax call, then make your PHP return JSON data and use JSON parser within GWT to parse your JSON.
Normally, when you create a GWT application, it runs with the default java server, to direct it to your php server, use the following,
noserver -remoteUI "${gwt_remote_ui_server_port}:${unique_id}"
-startupUrl <name>.html
-logLevel INFO -codeServerPort 9997
-war /opt/lampp/htdocs/../war com.<appname>.<appname>
if ever you are using eclipse, simply place the app on your php server you are using
Useful links:
and
Havent used, so I can't tell if it's any good