Karl has given the best answer if you are setting up a production-ready environment.
There are some known issues with later versions of PHP that we are currently addressing for future releases. Some fixes, such as issue #13307
(DIP upload from Archivematica fails with PHP 7.3+), were fixed before the 2.6.0 release, and the fixes are included. Other issues, such as #13397
(PHP 7.3+ causes the atom-worker to crash periodically), were identified post-2.6 release, and are issues we will be addressing in a future release.
To help us find and address these issues, and to better allow developers to assist us and report issues, we have just created a new Vagrant development environment that installs our qa/2.x code branch (i.e. where we will be adding new features and bug fixes for a 2.7 release) with PHP 7.4 and Ubuntu 20.04. We fully expect there to be issues, but hope that testing in this environment will help us identify them. See for example:
If you want a stable box in which to test and develop against AtoM 2.6, you would want to install the v22.214.171.124 box. If you are interested in seeing how AtoM runs with PHP 7.4 and Ubuntu 20.04 and you'd like to help us by reporting issues related to this that you discover through testing, then installing the latest v126.96.36.199 box will give you this.
If you would like to learn more about how to set up an AtoM Vagrant environment, please see: