socket_select方法如何工作？

Question

我需要你的帮助。

任何人都可以向我解释socket_select方法如何工作以及它如何影响它所采用的参数，即（读，写，除外）。

我有这个php示例，但我无法理解，所以我需要你的帮助

error_reporting(~E_NOTICE);
set_time_limit (0);

$address = "127.0.0.1";
$port = 5000;
$max_clients = 10;

if(!($sock = socket_create(AF_INET, SOCK_STREAM, 0)))
{
    $errorcode = socket_last_error();
    $errormsg = socket_strerror($errorcode);

    die("Couldn't create socket: [$errorcode] $errormsg \n");
}

echo "Socket created \n";

// Bind the source address
if( !socket_bind($sock, $address , 5000) )
{
    $errorcode = socket_last_error();
    $errormsg = socket_strerror($errorcode);

    die("Could not bind socket : [$errorcode] $errormsg \n");
}

echo "Socket bind OK \n";

if(!socket_listen ($sock , 10))
{
    $errorcode = socket_last_error();
    $errormsg = socket_strerror($errorcode);

    die("Could not listen on socket : [$errorcode] $errormsg \n");
}

echo "Socket listen OK \n";

echo "Waiting for incoming connections... \n";

//array of client sockets
$client_socks = array();

//array of sockets to read
$read = array();

//start loop to listen for incoming connections and process existing connections
while (true) 
{
    //prepare array of readable client sockets
    $read = array();

    //first socket is the master socket
    $read[0] = $sock;

    //now add the existing client sockets
    for ($i = 0; $i < $max_clients; $i++)
    {
        if($client_socks[$i] != null)
        {
            $read[$i+1] = $client_socks[$i];
        }
    }

    //now call select - blocking call
    if(socket_select($read , $write , $except , null) === false)
    {
        $errorcode = socket_last_error();
        $errormsg = socket_strerror($errorcode);

        die("Could not listen on socket : [$errorcode] $errormsg \n");
    }

    //if ready contains the master socket, then a new connection has come in
    if (in_array($sock, $read)) 
    {
        for ($i = 0; $i < $max_clients; $i++)
        {
            if ($client_socks[$i] == null) 
            {
                $client_socks[$i] = socket_accept($sock);

                //display information about the client who is connected
                if(socket_getpeername($client_socks[$i], $address, $port))
                {
                    echo "Client $address : $port is now connected to us. \n";
                }

                //Send Welcome message to client
                $message = "Welcome to php socket server version 1.0 \n";
                $message .= "Enter a message and press enter, and i shall reply back \n";
                $r= implode(",", $read)." ReadWriteExcept";
                $w = implode(",", $write)." ReadWriteExcept";
                $e = implode(",", $except)." ReadWriteExcept";

                socket_write($client_socks[$i] , $message." ".$r." ".$w." ".$e);

                break;
            }
        }
    }

    //check each client if they send any data
    for ($i = 0; $i < $max_clients; $i++)
    {
        if (in_array($client_socks[$i] , $read))
        {
            $input = socket_read($client_socks[$i] , 1024);

            if ($input == null) 
            {
                //zero length string meaning disconnected, remove and close the socket
                unset($client_socks[$i]);
                socket_close($client_socks[$i]);
            }

            $n = trim($input);

            $output = "OK ... $input";

            echo "Sending output to client \n";

            //send response to client
            socket_write($client_socks[$i] , $output." welcome any time");
        }
    }
}