Adding a function to previous class to connect with key instead of user/password
<?php
public function loginWithKey($username, $publicKey, $privateKey, $passphrase = null)
  {
    if (!@ssh2_auth_pubkey_file($this->connection, $username, $publicKey, $privateKey, $passphrase)) {
      throw new Exception("Could not authenticate with given keys or passphrase");
    }
    $this->sftp = @ssh2_sftp($this->connection);
    if (!$this->sftp) {
      throw new Exception("Could not initialize SFTP subsystem.");
    }
  }
?>