PHPSessionHandler registers SessionManager as a PHP session handler, which means that SessionManager methods are invoked (through PHPSessionHandler methods) at various points in the request lifetime. This is in theory only necessary for compatibility with extension code which uses PHP session handling ($_SESSION, session_start() etc), which hopefully doesn't happen anymore as it was deprecated a decade ago.
In practice though, I don't think disabling PHPSessionHandler was ever tried on any large wiki, so it needs some testing. Nevertheless, it's worth doing this in Wikimedia production - it makes the system much easier to reason about, and probably yields some performance improvement too.
We'd have to set $wgPHPSessionHandling = 'warn', run for a few weeks to see if there are any warnings, then set $wgPHPSessionHandling = 'disable' and watch another few weeks for errors. If all goes well, eventually we'll probably want to follow with an identical change in the MediaWiki default config as well.