WordPress recommends using PHP 7.3 [1] on web servers.

Our simple hosting solution instance on Gandi.net still uses PHP 5.6 and the upgrade to 7.3
can be easily done through the Gandi configuration interface.

I have locally tested that our main website can be correctly served using PHP 7.3 and everything still
works as expected.

Considering that the PHP version requirement will be surely bumped to 7 for WordPress 5 in the end
of that year [2], I think we should do that PHP upgrade soon and once for all.

[1] https://wordpress.org/about/requirements/
[2] https://make.wordpress.org/core/2018/12/08/updating-the-minimum-php-version/