From Joomla! Documentation
If you’ve logged into your Joomla Administrator area and have been greeted with a message like the one below, don’t panic. Joomla has a recommended minimum PHP requirement at the time of writing of PHP 7.3 or higher. The current official release of PHP is version 7.4. In addition to the PHP Version Check message that can appear if you’re using a version of PHP lower than 7.3, Joomla 3.10 will have a PHP warning message about PHP versions before you attempt to migrate to Joomla 4 when it is released.
With Joomla 4 a large amount of previous legacy code has been removed. At the same time, a lot of new modern code has been added. Having the correct version of PHP is going to be essential for Joomla 4 to work, as the new code utilises the improved features and functions PHP 7.3+ provides.
Technical Requirements for Joomla
There are a range of technical requirements recommended for hosting Joomla sites. The Production Department review the requirements from time to time, and so you should reference the Joomla Downloads site for current Technical Requirements.
Though the PHP minimum requirement is 5.3.10, many Joomla extensions additionally have features that may push up the minimum required version of PHP need. Should you get a white screen of death when trying to use Joomla on PHP 5.x, it is highly likely that an installed extension is not compatible with PHP 5.x and the easiest solution would be to upgrade your PHP to a currently supported version of PHP.
Updating your PHP Version
There's a variety of ways to update your PHP version. The main ones are covered in an article in the September 2020 issue of Joomla Community Magazine: How do I update my PHP version?.
If you have additional notes on updating your PHP Version, please contribute them here.
PHP is a server-level application layer. So this means that when something goes wrong, it really goes wrong, and most commonly brings your website down with a number of errors.
Over time I’ve only encountered a handful of errors when switching PHP versions, and the two most common (both which I triggered while creating screenshots for this article) are:
- 503 Error - Service Unavailable - A 503 error may occur temporarily while the server changes settings for your account. If it continues to display after a few minutes, revert the version change you made.
- 502 Error - Bad Gateway - 502 errors are a permanent issue when they occur. Usually, it means that the change you made is not compatible with the settings on the server. I triggered a 502 error when changing the Plesk version, as instead of keeping the “run PHP as” option set to what it was originally set to. Changing the setting to match the original setting cleared the 502 error.