Ubuntu – the gnome-open analog on XFCE desktop

I think xdg-open is the command you are looking for.

NAME
       xdg-open - opens a file or URL in the user's preferred application

SYNOPSIS
       xdg-open {file | URL}

       xdg-open {--help | --manual | --version}

DESCRIPTION
       xdg-open opens a file or URL in the user's preferred application. If a
       URL is provided the URL will be opened in the user's preferred web
       browser. If a file is provided the file will be opened in the preferred
       application for files of that type. xdg-open supports file, ftp, http
       and https URLs.

eg: xdg-open index.php

This will open index.php in gedit(if you are using gnome).

If you want to open a url in browser

xdg-open http://google.com

this will open google.com in your default browser.

xdg-open is a wrapper script - it will use the desktop environment's tool (gio open, gvfs-open, kde-open, gnome-open, dde-open, exo-open, and a host of other such tools). It is also installed by default, and very likely to work on past, current and future versions (on the other hand, gvfs-open and gnome-open have been deprecated, and may be unavailable in future releases).

I bet it was Nautilus taking over ! If it happens again terminate nautilus in task manager. If you want to switch to xfce permanently get rid of Nautilus in favor of Thunar or maybe PCMan.