PHPMailer\PHPMailer\SMTP::authenticate PHP Method

    public function authenticate($username, $password, $authtype = null, $OAuth = null)
    {
        if (!$this->server_caps) {
            $this->setError('Authentication is not allowed before HELO/EHLO');
            return false;
        }
        if (array_key_exists('EHLO', $this->server_caps)) {
            // SMTP extensions are available. Let's try to find a proper authentication method
            if (!array_key_exists('AUTH', $this->server_caps)) {
                $this->setError('Authentication is not allowed at this stage');
                // 'at this stage' means that auth may be allowed after the stage changes
                // e.g. after STARTTLS
                return false;
            }
            $this->edebug('Auth method requested: ' . ($authtype ? $authtype : 'UNKNOWN'), self::DEBUG_LOWLEVEL);
            $this->edebug('Auth methods available on the server: ' . implode(',', $this->server_caps['AUTH']), self::DEBUG_LOWLEVEL);
            if (empty($authtype)) {
                foreach (['LOGIN', 'CRAM-MD5', 'NTLM', 'PLAIN', 'XOAUTH2'] as $method) {
                    if (in_array($method, $this->server_caps['AUTH'])) {
                        $authtype = $method;
                        break;
                    }
                }
                if (empty($authtype)) {
                    $this->setError('No supported authentication methods found');
                    return false;
                }
                $this->edebug('Auth method selected: ' . $authtype, self::DEBUG_LOWLEVEL);
            }
            if (!in_array($authtype, $this->server_caps['AUTH'])) {
                $this->setError("The requested authentication method \"{$authtype}\" is not supported by the server");
                return false;
            }
        } elseif (empty($authtype)) {
            $authtype = 'LOGIN';
        }
        switch ($authtype) {
            case 'PLAIN':
                // Start authentication
                if (!$this->sendCommand('AUTH', 'AUTH PLAIN', 334)) {
                    return false;
                }
                // Send encoded username and password
                if (!$this->sendCommand('User & Password', base64_encode("" . $username . "" . $password), 235)) {
                    return false;
                }
                break;
            case 'LOGIN':
                // Start authentication
                if (!$this->sendCommand('AUTH', 'AUTH LOGIN', 334)) {
                    return false;
                }
                if (!$this->sendCommand("Username", base64_encode($username), 334)) {
                    return false;
                }
                if (!$this->sendCommand("Password", base64_encode($password), 235)) {
                    return false;
                }
                break;
            case 'XOAUTH2':
                //If the OAuth Instance is not set. Can be a case when PHPMailer is used
                //instead of PHPMailerOAuth
                if (is_null($OAuth)) {
                    return false;
                }
                $oauth = $OAuth->getOauth64();
                // Start authentication
                if (!$this->sendCommand('AUTH', 'AUTH XOAUTH2 ' . $oauth, 235)) {
                    return false;
                }
                break;
            case 'CRAM-MD5':
                // Start authentication
                if (!$this->sendCommand('AUTH CRAM-MD5', 'AUTH CRAM-MD5', 334)) {
                    return false;
                }
                // Get the challenge
                $challenge = base64_decode(substr($this->last_reply, 4));
                // Build the response
                $response = $username . ' ' . $this->hmac($challenge, $password);
                // send encoded credentials
                return $this->sendCommand('Username', base64_encode($response), 235);
            default:
                $this->setError("Authentication method \"{$authtype}\" is not supported");
                return false;
        }
        return true;
    }