Last week I described in the article How to set up NetBeans to treat files with an extension as PHP files, how to do it in the Early Access build and I mentioned that the NetBeans core team is working on simplifying the procedure. And now the feature is in the build.
If you download a continual or daily build, you can see the Open As ... action in the context menu of files that are not recognized by default. If you invoke it, then the Open As ... dialog opens and you can choose how the files with the same extension will be treated in NetBeans. If you choose mimetype text/x-php5, then files with the same extension will be opened in the PHP editor.
And you can change it later in the Options dialog.