Shared codebase with different UIs

Two general ways

• Structure the shared parts as Composer packages

For instance, all the application level code, such as router, dependency managing, validation, domain entities mapping can be managed as Composer dependencies through Packagist. It's possible to use private repos as dependencies if code is not open sourceable.

You would then have a different repo for each site, and make the parts which are the same be built using Composer, and only coding the part which is actually different. You'd probably have two servers, for different domains.

This implies more configuration through Composer and code.

repo 1
    /app
        /viewSpecificToOne
    /public
        /cssSpecificToOne
        /jsSpecificToOne
        index.php
    /vendor
        /[dependencies installed through composer]
repo 2
    /app
        /viewSpecificToTwo
    /public
        /cssSpecificToTwo
        /jsSpecificToTwo
        index.php
    /vendor
        /[dependencies installed through composer]

• Manage all the sites from the same repo

This is close to the other solution, but you would send the different routes to different front Controllers, who would use the same domain mappers, validation, routers, etc. but have two different View layers, that would feature specific code.

This implies more configuration from the server to serve each domain from the two public repos.

Repo
    /app
        /view1
            /viewLogicSpecificToOne
        /view2
            /viewLogicSpecificToTwo
    /public1
        /cssSpecificToOne
        /jsSpecificToOne
        index.php
    /public2
        /cssSpecificToTwo
        /jsSpecificToTwo
        index.php
    /lib
        /sharedPackages