PHP 8.1

We should support PHP 8.1.

The deprecations and breaking changes are listed in the release notes at https://www.php.net/releases/8.1/en.php#deprecations_and_bc_breaks

Here are the backward compatibility issues:

1. Tentative return types in PHP built-in class methods
2. HTML entity en/decode functions process single quotes and substitute by default
3. $GLOBALS variable restrictions.
4. MySQLi: Default error mode set to exceptions

I am most worried about HTML encode/decode.

[ ] create edge test setup for PHP 8.1