I would do this way:
1) Have a text file available on the remote server containing just the file version of the new file.
A text file as the advantage that (as astions mentionned) it will work even if remote includes are disabled. The file could be generated from a PHP script if needed.
2) The local script would get the remote version file number at startup, but only once in a while. For exemple, once per day or once per week. That is because if the script checks for updates at every hit, it will 1) slow down the script a lot 2) overload the remote server if your script is popular.
On the oppsite to Todd_Z, I prefer the idea of the script deciding itself, from the version number received, if it needs to receive an update or not, instead of the remote server deciding if the script has to update or not. No particular reason beside I find that simpler, on first sight.
Hope this helps,
Tom
