If you upgraded your Joomla 3 site to Joomla 4, and everything is working, congratulations! If not, try to read some of the processes in my previous article. However, even after a successful upgrade, you can observe some errors in your Joomla Back End. One of the common ones I saw was missing "categories" from the "Contacts" menu within the Component section of the Joomla administration Back End. You can observe just one menu item missing or more, and not only by "Contacts" but as well by other menu items. The cause of this is an exception during the SQL queries run during the upgrade that is preventing to add, moving, etc... the items within the database table.
After updating the PHP to 8.1 or PHP 8.2 it is time to upgrade as well the Joomla version from the latest stable version Joomla 3.10.11 Stable to Joomla 4. This process is usually pain but you need to watch out for incompatible plugins and modules. During the PHP update, we had the Back End still accessible, however, during upgrading to Joomla 4 an incompatible plugin or module or component can cause the Back End to be as dead as the Front End. This can cause a lot of issues since uninstalling the plugin after can be a pain. We will address this issue and try to fix the extension until the moment we can at least uninstall it. However, I recommend uninstalling anything that Joomla 4 upgrade script will point as a risk, you will save yourself a head ache.
Many websites are still running on older versions of PHP, usually PHP 7.4 or even older, and this can represent a security and performance risk for the website and therefore is always recommended to keep with the current version of PHP. For example, PHP 7.4 is already End Of Life to the 18 Nov 2021, this means that there are no security or code updates whatsoever. Someone could say that he do not care, which is an option, and since the website that runs the older version of PHP is not ciritical, even vulnurable security is acceptable. However, what if you would like to keep with Joomla 4? The recomended PHP version is 8.0 although the minimum still supported is PHP 7.2.5. This give us one more reason to not upgrade since you can still use the benefit of Joomla 4 on older PHP version but newer PHP means as well a performance boost and supported new features. Hence, it is more than recommended to move and upgrade the PHP to ensure the best exprience. Just look on the release note:
PHP 8.1 is a major update of the PHP language that brings better performance, better syntax, and improved type safety. It also contains new features and optimization improvements.