For the PHP-side application developer, PHP is in 99% of use cases shared-nothing no matter the hosting environment - CGI, FCGI, FPM, mod_php. The exceptions are persistent socket (database) related, obviously you don't have these in a CGI environment.

The mod_php "share-something" question was always more relevant for ops people.

Obviously it's share something e.g. session state and database state.

But architecture of PHP applications is simple and it works. I like simple.