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 .html
-logLevel INFO -codeServerPort 9997
-war /opt/lampp/htdocs/../war com..
if ever you are using eclipse, simply place the app on your php server you are using