I spent some time trying to integrate Mantis to SMF using the APIs, but it was an adventure in annoyance and failure. So I decided to bypass the whole thing entirely, let Mantis keep its own accounts database, but modify authentication_api.php to use data from the smf_members table to validate logins. If the account is valid in SMF but doesn't exist in Mantis, it calls user_signup to create it. Make sure you read the commit information, as you might need to edit in the right SMF table name.
Commit: https://github.com/leandrotlz/mantisbt/ ... 265f88851e
Fork: https://github.com/leandrotlz/mantisbt
Watch that fork if you are interested in future tweaks and fixes.
How to log in to Mantis with SMF accounts
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