Description:
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From manual page: http://www.php.net/function.password-hash
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According to the relevant RFC at [1] Argon2i via password_hash() is not only optional but needs extra work for inclusion into the respective PHP installation unless one uses a precompiled build with everything inside (e.g. PHP's official Windows builds), this leads to it being unavailable at some webhosters which, while not being really great, leads to a whole different problem.

one cannot expect Argon2i to be available just because PHP is 7.2 (and Argon2id on 7.3 respectively), meaning users need to know that the PHP Version is NOT the correct way to judge Argon2 availability, and that they will need to check for it even if the PHP is new enough, and either fail gracefully or at least fall back to default.

[1]
https://wiki.php.net/rfc/argon2_password_hash