I encounter that kind of problem some time and again, too. Try deleting the file. If the browser still shows "Hello world", I know of these reasons:
- You saved into a local directory instead of into the WAMPP directory. E.g. sometimes I open a file from WebDAV and accidentially save it to the temp folder where it was downloaded to.
- Your browser is looking at another directory / file than the one you're saving to.
- Some serious caching is happening in your connection from editor - filesystem - WAMPP - browser. This would disqualify your system for coding php, if you can't get rid of the caching.
Hope my (negative) experiences bring a positive result for you. ;)