SMTP::authenticate( string $username, string $password, string $authtype = null, PHPMailer\PHPMailer\OAuthTokenProvider $OAuth = null ): bool

518    public function authenticate(519        $username,520        $password,521        $authtype = null,522        $OAuth = null523    ) {524        if (!$this->server_caps) {525            $this->setError('Authentication is not allowed before HELO/EHLO');526 527            return false;528        }529 530        if (array_key_exists('EHLO', $this->server_caps)) {531            //SMTP extensions are available; try to find a proper authentication method532            if (!array_key_exists('AUTH', $this->server_caps)) {533                $this->setError('Authentication is not allowed at this stage');534                //'at this stage' means that auth may be allowed after the stage changes535                //e.g. after STARTTLS536 537                return false;538            }539 540            $this->edebug('Auth method requested: ' . ($authtype ?: 'UNSPECIFIED'), self::DEBUG_LOWLEVEL);541            $this->edebug(542                'Auth methods available on the server: ' . implode(',', $this->server_caps['AUTH']),543                self::DEBUG_LOWLEVEL544            );545 546            //If we have requested a specific auth type, check the server supports it before trying others547            if (null !== $authtype && !in_array($authtype, $this->server_caps['AUTH'], true)) {548                $this->edebug('Requested auth method not available: ' . $authtype, self::DEBUG_LOWLEVEL);549                $authtype = null;550            }551 552            if (empty($authtype)) {553                //If no auth mechanism is specified, attempt to use these, in this order554                //Try CRAM-MD5 first as it's more secure than the others555                foreach (['CRAM-MD5', 'LOGIN', 'PLAIN', 'XOAUTH2'] as $method) {556                    if (in_array($method, $this->server_caps['AUTH'], true)) {557                        $authtype = $method;558                        break;559                    }560                }561                if (empty($authtype)) {562                    $this->setError('No supported authentication methods found');563 564                    return false;565                }566                $this->edebug('Auth method selected: ' . $authtype, self::DEBUG_LOWLEVEL);567            }568 569            if (!in_array($authtype, $this->server_caps['AUTH'], true)) {570                $this->setError("The requested authentication method \"$authtype\" is not supported by the server");571 572                return false;573            }574        } elseif (empty($authtype)) {575            $authtype = 'LOGIN';576        }577        switch ($authtype) {578            case 'PLAIN':579                //Start authentication580                if (!$this->sendCommand('AUTH', 'AUTH PLAIN', 334)) {581                    return false;582                }583                //Send encoded username and password584                if (585                    //Format from https://www.rfc-editor.org/rfc/rfc4616#section-2586                    //We skip the first field (it's forgery), so the string starts with a null byte587                    !$this->sendCommand(588                        'User & Password',589                        base64_encode("\0" . $username . "\0" . $password),590                        235591                    )592                ) {593                    return false;594                }595                break;596            case 'LOGIN':597                //Start authentication