I know PHP include is server sided, but is there something similar I can implement that can work across different servers? Or something server sided that can work on the same server but across different URLs?
An include for a URL is possible. the php.ini file has a parm allow_url_include that is normally set to off for security reasons. You should be able to set it on with a local php.ini file (to over-ride the system php.ini file) assuming that your web host hasn't done anything to prevent that. There is also an ini_set command in php that may allow you to enable it from your program (unless somehow blocked by your web host). As mentioned, it does open the door to some security issues so do your research.