It is possible, but if it is a .php file and that server runs PHP, then when you include it all you will get is the output of that file when you include it, not the actual PHP code like you do when you include a file locally. You can get around this by naming the file with a suffix such as .txt or otherwise configuring the server to serve it up as plain text without any other processing.
However, be aware that this can be a security hazard. If that one file is somehow compromised, every script that includes it will now be at risk of executing any code injected into or replacing that file; and perhaps more of an issue is that the file will, by definition, have to accessible to the internet, and that might not be very desirable.
Also, if you do not have control over the host configuration, you may not be able to do it anyway since it is common to disable the allow_url_include option.
