detect if running a unit test

Is there a way for a function to detect if it is being run by a unit test or not? (and not on a prod web server, for example)

Yes, the usual approach is to set an environment variable, a global variable, or a constant in phpunit.xml configuration file (doc at https://phpunit.de/manual/6.5/en/appendixes.configuration.html)

Add the following section into phpunit.xml' root <phpunit> element to define constant IS_TESTING:

<php>
  <const name="IS_TESTING" value="1"/>
</php>

then in your PHP code, you can do the following:

if (defined('IS_TESTING')) { ... }