Collective work on the site

Not all sites are made by the studio and then given to the customer.

It happens that complex sites of portal type develop in stages,
part is available online, another is under development,
moreover, in fairly large projects several independent groups can work on the site.

Here and without binoculars it can be seen that it would be nice to use a repository for storing php and html code.
')
But how to do it right?
With designers and programmers, everything is quite simple, they develop code on local machines and make changes to the repository. You can even configure cron to periodically update the latest changes from the repository to the online site.

But what about content?
The content changes only on the online version, in order not to complicate the matter by synchronizing data from different databases. But not all the content is stored in the database, often the CMS content of static pages is stored in .php files, which causes new files and changes, which periodically have to be transferred from the online to the repository, and do it manually to avoid version conflicts.

I wonder who faced and how to solve such problems.