handling multiple connections code

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networking

sockets

I need your help.

Please I need to understand some lines in this code which about handling multiple connections and how they effect the code.

error_reporting(~E_NOTICE);
set_time_limit (0);

$address = "0.0.0.0";
$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";
				$message .= "Enter a message and press enter, and i shall reply back \n";
				socket_write($client_socks[$i] , $message);
				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);
		}
	}
}

the lines which I can't understand are:

the first line is $read[0] = $sock; here I can't understand why the coder store the master socket in the read array.

the second line is if (in_array($sock, $read)) here I can't understand how he/she check for a new connection using this line and how he/she check for the master socket if it is exist in the read array while he/she store it already in the read array.

the last line is if (in_array($client_socks[$i] , $read)) here I can't understand why the coder check for the client socket if it is exist in the read array while he/she want to write for it.

thanks every body.