Description:
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it's slow that phpscript itself have to loop through $fds and use in_array to know which client is sending data

$allfds[0] = socket_create(...);
$clients[0] = new MyClient($allfds[0]);
...

$count = socket_select($fds);
for ($i = 0; $i < FDSIZE; $i ++) {
  if (in_array($fds, $allfds[$i])) {
    $clients[$i]->processIncomingData();
  }
}
larger FDSIZE make script slower

i'd suggest $fd resource store more information: ID
int socket_set_id(source $sock)
int socket_get_id(source $sock)

$allfds[0] = socket_create(...);
$clients[0] = new MyClient($allfds[0]);
socket_set_id($allfds[0], 0);
$allfds[1] = socket_create(...);
$clients[1] = new MyClient($allfds[1]);
socket_set_id($allfds[1], 1);

$count = socket_select($fds);
for ($i = 0; $i < $count; $i ++) {
  $id = socket_get_id($fds[$i]);
  $clients[$id]->processIncomingData();
}
speed is not depend on FDSIZE