PHP is the engine your site runs on. New versions are faster and safer; old code sometimes needs the version it was written for.
Switching
Site → PHP tab → pick a version → apply. It takes effect within a minute, and you can switch back just as easily.
Which version?
- New project / up-to-date WordPress: the newest offered.
- Older app that breaks on the newest: step down one version at a time until it runs, then plan an update — old PHP versions eventually lose security support.
If the site errors after switching
Check the Logs tab — a fatal mentioning deprecated or undefined function right after a switch means some code isn't ready for that version. Switch back and update the code or plugin first.