Actually, this is a very difficult decision. Anyone responding with 7.x+; you do know PHP 7.x is EOL and no longer supported.
manu 7.4 + only (or newer), It is a security nightmare to manage unsupported versions of PHP
I agree this poses a security risk having older PHP versions installed. Older versions of PHP is now anything below 8.0.
@Adam could there be (or would there be) any possibility of the server admin installing older versions somehow? PHP 5.6 might be out of date, but I still have clients on this version who refuse to upgrade, but I also use PHP 5.6 for testing purposes for software releases too because of this single fact:
20%~ is on PHP 5.x - reference: https://w3techs.com/technologies/details/pl-php that's a large number.