Use of Undefined Constant Error in PHP

I have 3 environments in which I am working… development, test and production.

My new user signup procedure works perfectly in development and test.

In my production environment, it is throwing an error:

file: "/var/app/current/dmxConnectLib/lib/formatters/crypto.php"
line: 58
message: "Use of undefined constant PASSWORD_ARGON2ID - assumed 'PASSWORD_ARGON2ID' (this will throw an Error in a future version of PHP)"

This is on AWS using PHP 7.3 running on 64bit Amazon Linux/2.9.7.