PHP socket connect from external IP problem

i have a problem with PHP Socket connect on an ubuntu-server with PHP7 behind NAT. I can connect from all devices in local-network (192.168.2.XXX) with telnet. But i cannot connect from external IP from Internet. PORT 5555 is open in firewall and i can see traffic coming in with "tcpdump". but PHP is ignoring this. Here is my code so far:

$port = 5555;
$socket = socket_create(AF_INET,SOCK_STREAM,SOL_TCP) or die("socket_create failed");
socket_bind($socket,'0.0.0.0',$port) or die("socket_bind failed");
socket_listen($socket);

while(true)
{
    if(($newc = socket_accept($socket)) !== false)
    {
                echo "connected ...";
                socket_close($newc);
    }
}
socket_close($socket);

Thanks ahead for your help...

Dave