Is it possible to upgrade system PHP provided by OS vendor to the next version on a Plesk server?
If this is a server with Plesk Obsidian
On a fresh Plesk Obsidian, Plesk services do not depend on system PHP, which makes it possible to upgrade system PHP to the next version.
If this is a server with Plesk Onyx or previous release
It is not recommended to upgrade system PHP as Plesk and webmail depend on it. The upgrade should be performed at your own risk (a pre-upgrade server snapshot / backup is always recommended).
Warning: Before upgrading system PHP, change PHP for webmail using the steps from this KB article.