Fix national checksum calculation for Belgium

[Olympic1]

The function never validated the national checksum for Belgium correctly. What it did was:

• Get the checksum from the IBAN
• Remove the bankID from the BBAN to get the account
• Get the checksum from the account
• Calculate the checksum with mod 97
• Check if the calculated checksum is the same as the checksum from the IBAN (Which will always be true)

I changed it so that it now calculates it following the correct rules.

• Get the checksum from the IBAN
• Get the BBAN from the IBAN
• Remove the checksum from the BBAN
• Calculate the checksum from the left over BBAN with mod 97
• Check if the calculated checksum is the same as the checksum from the IBAN

Here is a script with the current and new code
https://onlinephp.io/c/60fa7

[globalcitizen]

Great catch! What a horrible bug. Thanks for a well documented pull request. The tests currently only verify that a set of example pass, they cannot catch logic errors where the verification code always returns true. I will run a round of further tests on the suite of examples just to verify they all work and modifications fail, then push this out.

[globalcitizen]

Released as v4.1.1. Thanks again!

In case you are interested, we could probably do with:

• A check of all the other pre-IBAN national checksum algorithms to ensure no similar bugs exist for those implementations.
• Some new implementations documented over here

[Olympic1]

Ok, I'll give it a look.