Many may ask why would you need to do this? When upgrading from phpBB 2 to 3 the conversion does not keep the admin user you create. If your old admin user has an email address associated that no longer works or is accessible this makes it impossible to reset the admin password.
Some may just say "I'll just manually edit the e-mail address" but it won't work and phpBB will fail to find that user if you don't update the "user_email_hash" field but how do you do that?
Use this PHPcode:
echo sprintf('%u', crc32(strtolower($email))) . strlen($email);
It will output the corresponding hash you need to use.
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