The latest version of Deskpro requires PHP v5.5 or newer.
For significant performance benefits, we recommend using PHP 7.4
If you receive an error message during installation that you are using an older version, the only way to fix the error is by upgrading PHP.
The process of upgrading PHP varies greatly depending on your server. You can always get the latest version from http://php.net/downloads.php and installation instructions are available in the PHP manual at http://php.net/manual/en/install.php
Note that most servers have installed PHP packages rather than PHP from source that you can get from the PHP website. Here is a list of further reading for a variety of platforms: