PHP 7.3

[brandonramirez]

Note that PHP 7.2 and 7.3 show tons of warnings about null not being Countable, however, they do not break the build.

It's a known issue in this version of PHPUnit. There is a fix, but the maintainer of PHPUnit is not interested in back-porting to PHPUnit 4.x or even 5.x.
sebastianbergmann/php-code-coverage#554

PHPUnit 4.x PR: sebastianbergmann/php-code-coverage#590
PHPUnit 5.x PR: sebastianbergmann/php-code-coverage#591

So in order for tests to run perfectly clean under PHP 7.2 and 7.3, we need to upgrade PHPUnit to at least 6.x, which kills support for PHP 5.x (minimum supported: PHP 7.0)

I think we can live with the warnings for the time being, but should consider dropping support for older PHP versions in the near future.

[mpetrovich]

I agree about dropping pre-7.x support. For simplicity and maintainability, we should start following the official PHP version support schedule: https://www.php.net/supported-versions.php

Interestingly, this would mean dropping support for 7.0 immediately and for 7.1 in January.