Users should be aware of bug 78661 https://bugs.php.net/bug.php?id=78661
As of december 2020, on some Linux distro, (debian 10 and ubuntu 20.04 at least), although the ssh daemon will accept the connection, the function ssh2_auth_pubkey_file stubbornly reports an error.
It took me hours to realise that my code was correct, but the function buggy. If you are hit by this bug, you may try https://www.php.net/ssh2_auth_password instead... If applicable.