It seems like $json_string defined by \wp_json_encode($payment...st, \JSON_PRETTY_PRINT) on line 41 can also be of type false; however, PHPUnit\Framework\Assert...nStringEqualsJsonFile() does only seem to accept string, did you maybe forget to handle an error condition?
This check looks for type mismatches where the missing type is false. This
is usually indicative of an error condtion.
This function either returns a new DateTime object or false, if there was an error.
This is a typical pattern in PHP programming to show that an error has occurred without
raising an exception. The calling code should check for this returned false
before passing on the value to another function or method that may not
be able to handle a false.
It seems like the type of the argument is not accepted by the function/method which you are calling.
In some cases, in particular if PHP’s automatic type-juggling kicks in this might be fine. In other cases, however this might be a bug.
We suggest to add an explicit type cast like in the following example: