Authenticate using a public key read from a file.
An SSH connection link identifier, obtained from a call to ssh2_connect.
The public key file needs to be in OpenSSH's format. It should look something like:
ssh-rsa AAAAB3NzaC1yc2EAAA....NX6sqSnHA8= rsa-key-20121110
If privkeyfile is encrypted (which it should be), the passphrase must be provided.
Returns true on success or false on failure.
The underlying libssh library doesn't support partial auths very cleanly That is, if you need to supply both a public key and a password it will appear as if this function has failed. In this particular case a failure from this call may just mean that auth hasn't been completed yet. You would need to ignore this failure and continue on and call ssh2_auth_password in order to complete authentication.