At my previous company we had a shared $_SERVER['shared_code_dir'] directory that was usable by all our sites on our server. I know in php you can't use the include() function with an https web address for security reasons. But we had a directory set up that could be used securely. Does anyone know how to set that up? I apologize if this isn't completely clear.
Although it's not recommended for security reasons, particularly if it's an internet-facing website because it might get abused if coded improperly, it can be configured!
You'll need to enable allow_url_include in your php.ini configuration for it to work.