This doesn't explain the phpstan bugs, see #13663

For a given set of PHP files, phpstan should always produce the same results, it doesn't! Until then, I suggest we disable phpstan, or come up with a proper fix!

Besides, your change to the model/extension/payment/opayo.php for the sendCurl is wrong! phpstan needs to be fixed if it can't cope with functions using optional parameters.

This has noting to do with optional parameters and isn't a bug in PHPStan. It's caused by there being two classes with the same name that both have a method called sendCurl(). One has 3 arguments the other 2. So it depends on which one is processed first and there for will give different results on different computers.
This issue was solved in OC4 by using namespace.

Wrong. In the case of the sendCurl methods, they are defined in differently named classes (16 of them in OC 3.0.x.x), and can have therefore different method signatures.

Please undo your changes!

Sorry, you are wrong. Different classes in PHP, using the same name for a method, can use different method signatures for this. These are not bugs in OC 3.0.x.x! And developers shouldn't have to worry about how they name class methods, with what signatures, as long the methods are correctly used and called upon.

I reported this bug here: phpstan/phpstan#10560 on the phpstan repository, though I don't have time to write more sample codes for this on their repo. Busy making OC 3.0.x.x stable again!