[TimWolla]

Disclosure: I am not the author of the initial bug, but I investigated the issue together with another developer. I'm the author of the linked advisory and I've authored the commit that fixed the issue.

As the advisory states I don't know about the why, but I have a suspicion. PHP initially didn't implement BCrypt itself, but delegated to the system crypt, making the behavior of crypt() system-dependent. Now the PHP manual for crypt() showcases this example:

    crypt('rasmuslerdorf', '$2a$07$usesomesillystringforsalt$');

which uses a horrible salt that incidentally ends with a dollar sign. I suspect to keep compatibility for users that thought the dollar sign would be necessary at the end of the salt, the “PHP Hack” was included.

In fact such broken hashes appear to actually exist in the wild as showcased by this Stack Overflow question: https://stackoverflow.com/q/75519073/782822