This was detected as part of MDL-73587 (PHPCompatibility complete run against codebase).

While installing the MongoDB PHP library with composer guarantees that the get_debug_type() function will be available via PHP 8.0 polyfill, we don't install the library via composer, but copy on PSR4-enabled dir.

So, obviously, if that code is executed (out from PHPUnit, because PHPUnit also installs a polyfill) it will fail with PHP 7.4.

The complete error message is:

The function get_debug_type() is not present in PHP version 7.4 or earlier

It has been reported upstream:

• right now: https://jira.mongodb.org/plugins/servlet/mobile#issue/PHPLIB-1075 for more info.
• some months ago: https://github.com/mongodb/mongo-php-library/issues/897

And these are the current 2 cases in core:

• cache/stores/mongodb/MongoDB/Exception/InvalidArgumentException.php : 60
• cache/stores/mongodb/MongoDB/Exception/ResumeTokenException.php : 33

We need to replace them with old code-compatible-solution (what the polyfill does, basically).

And, maybe (apart) consider if, for 4.2 we ditch completely this store from core (MDLSITE-7106):

• It was created only as a reference-implementation.
• it's orders of magnitude slower than other caches (I don't think anybody in the world is using it in prod - though... I may be wrong).
• The library is giving us problems on each version (see this, see MDL-77095 ... and many other in the tracker along the last years).
• The php extension is a pain to build, it accounts ~ 30%-40% of the total time needed to build a PHP docker image.
• ...

Ciao