Steps to reproduce the issue

Call this running under PHP 8.1+:

$options = array('timeout' => 30);
$host = 'ftp.immowelt.net';
$port = '21';

$ftp = FtpClient::getInstance($host, $port, $options, null, null);
if (!$ftp->isConnected()) {
    exit('fail');
}
else {
    exit('success');
}

Expected result

success

Actual result

fail

System information (as much as possible)

Joomla 4.3
PHP 8.1

Additional comments

Joomla creates a connection:

$this->_conn = @ftp_connect($host, $port, $this->_timeout);

and checks it:

if (\is_resource($this->_conn)) {
   return true;
}

Since PHP 8.1 ftp_connect returns an [FTP\Connection] instance. Previously, a [resource] was returned.
https://www.php.net/manual/en/function.ftp-connect.php