For my current project we are commnunicating with an old mainframish hospital information system. This system has a service called 'Facelink' which is a simple TCP connection with a fixed set of commands. The problem is that a user can only be logged in one time. As we're hosting a web frontend with a lot of links to this service and we have some long running jobs running on the backend this can create concurrency issues (user already logged in).To counter this I've created a proxyserver class that is the only thing connected to the Facelink service and my jobs and scripts can connect to this as much as they want. It does what it should but as I am the only programmer working on this I have the nagging feeling that it can be written better. Please give me some tips and pointers.One question i already have is this:The protocol works a bit like this [COMMANDCODE] [MESSAGE], for example 001 User Identification = 1234. I'd like to get rid of the magic numbers and create constants, but since I have a FacelinkConnection class (used to connect to the FL service, now to the proxy), a FacelinkProxyServer class, and a bunch of FacelinkServiceX classes I wouldn't know where to put these. I've heard that a Facelink class with only constants is bad practice.Thanks for helping and sharing.class FacelinkProxyServer{    /**     * List of client sockets     * @var array     */    protected $clients        = array();    /**     * The socket clients will connect to     * @var resource     */    protected $listenSocket   = null;    /**     * The client socket that connects to the facelink server     * @var resource     */    protected $facelinkSocket = null;    /**     * The port we listen on for new connections     * @var int     */    protected $listenPort     = 8000;    /**     * The IP address we bind to     * @var string     */    protected $listenHost     = '127.0.0.1';    /**     * Are we are shutting down (stop accepting new clients)     * @var boolean     */    protected $shutdown       = false;    /**     * Create a new instance of FacelinkProxyServer     * @param array $facelinkConfig     * @param string $bindAddress     * @param int $bindPort     * @return FacelinkProxyServer     */    public function __construct (array $facelinkConfig, $bindAddress = '127.0.0.1', $bindPort = 8000)    {        $this->listenPort = $bindPort;        $this->listenHost = $bindAddress;        $this->createFacelinkConnection($facelinkConfig);        $this->createListenSocket();    }    /**     * Connects to the facelink service     * @param array $facelinkConfig     * @return void     */    protected function createFacelinkConnection(array $facelinkConfig)    {        $this->facelinkSocket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);        socket_set_block($this->facelinkSocket);        $timeout = 10;        $time = time();        while (!@socket_connect($this->facelinkSocket,                                 $facelinkConfig['host'],                                 $facelinkConfig['port']))        {          if(is_resource($this->facelinkSocket))            break;          /**           * Some sockets first return a error 115, then a error 114 to tell you connecting is in progress           */          $errorCode = socket_last_error($this->facelinkSocket);          if ($errorCode == 115 || $errorCode == 114)          {            if ((time() - $time) >= $timeout)            {              socket_close($this->facelinkSocket);              $this->shutdown();              break;            }            usleep(500000);            continue;          }        }        if(is_resource($this->facelinkSocket))            $this->facelinkLogin($facelinkConfig['user'], $facelinkConfig['password']);    }    /**     * Logs in to the facelink service     * @param $user     * @param $password     * @return void     */    protected function facelinkLogin($user, $password)    {        $this->readFacelinkSocket();        $this->writeFacelinkSocket(001 User Identification = .$user);        $this->readFacelinkSocket();        $this->writeFacelinkSocket(002 User Password = .$password);        $result = $this->readFacelinkSocket();        if(substr($result, 0, 3) == Facelink::CODE_USER_ALREADY_LOGGED_IN)            $this->shutdown();    }    /**     * Closes the proxy for new clients     * @return void     */    protected function shutdown()    {        @socket_close($this->listenSocket);        $this->shutdown = true;    }    /**     * Closes the connection to the facelink service     * @return void     */    protected function quit()    {        @socket_close($this->facelinkSocket);    }    /**     * Starts the listening socket for clients to connect to     * @return void     */    protected function createListenSocket()    {        $this->listenSocket = @socket_create(AF_INET, SOCK_STREAM, 0);        socket_set_nonblock($this->listenSocket);        socket_set_option($this->listenSocket, SOL_SOCKET, SO_REUSEADDR, 1);        $bindResult = @socket_bind($this->listenSocket, $this->listenHost, $this->listenPort);        if(!$bindResult)            $this->shutdown();        @socket_listen($this->listenSocket);    }    /**     * Handles a request from the client to the proxy     * @param int $connectionId The connection that sent the command      * @return void     */    protected function proxyCommand($connectionId)    {        $clientConnection = $this->clients[$connectionId];        $clientMessage = $this->readSocket($connectionId);        /**         * 003 is the logout command, swallow this.         */        if(substr($clientMessage, 0, 3) === 003)        {            $this->closeClientSocket($connectionId);            $this->removeSocket($connectionId);        }        /**         * Fake the clients login         */        elseif(in_array(substr($clientMessage, 0, 3), array('001', '002')))            $this->writeClientSocket($connectionId, '999 OK', false);        /**         * Proxy remote shutdown         */        elseif(substr($clientMessage, 0, 3) == 'DIE')            $this->shutdown();        elseif($clientMessage)        {                       $this->writeFacelinkSocket($clientMessage);            $this->pipeResponseToClient($connectionId);        }    }    /**     * Sends the response from the facelink service to the client, don't buffer anything.     * @param int $connectionId The connection that sent the original command     * @return void     */    protected function pipeResponseToClient($connectionId)    {        while(false !== @socket_recv($this->facelinkSocket, $data, 1024, 0))        {            if(!is_null($data))                $this->writeClientSocket($connectionId, $data);            if(ord(substr($data, -1)) == 26 || ord(substr($data, -1)) == 0 || is_null($data))            {                break;            }        }    }    /**     * Close the client's connection     * @param int $connectionId The connection to close     * @return unknown_type     */    protected function closeClientSocket($connectionId)    {        $clientConnection = $this->clients[$connectionId];        $this->writeClientSocket($connectionId, '999 BYE');    }    /**     * Removes a client socket from the pool     * @param int $connectionId The connection that is to be removed     * @return void     */    protected function removeSocket($connectionId)    {        $clientConnection = $this->clients[$connectionId];        @socket_shutdown($clientConnection, 2);        @socket_close($clientConnection);        unset($this->clients[$connectionId]);    }    /**     * Sends a message to a client     * @param int $connectionId The connection to sent the command to.     * @param string $message The message to send     * @param boolean $fromFacelink If we are sending a response from the facelink service     * @return void     */    protected function writeClientSocket($connectionId, $message, $fromFacelink = true)    {        if(!is_resource($this->clients[$connectionId]))        {            $this->closeClientSocket();        }        /**         * Facelink already attaches the chr(26) to the end of the command, the proxy         * needs to emulate this behaviour.         */        if(!$fromFacelink)        {            $message .= \n.chr(26);        }        $connection = $this->clients[$connectionId];        @socket_write($connection, $message, strlen($message));     }    /**     * Write a message from a client to the facelink service     * @param string $message The message to send     * @return void     */    protected function writeFacelinkSocket($message)    {        if(empty($message))            return;        $message .= chr(26);        socket_write($this->facelinkSocket, $message, strlen($message));    }    /**     * Reads information from a client socket     * @param int $connectionId The connection to read from     * @return string Information from the client     */    protected function readSocket($connectionId)    {        $connection = $this->clients[$connectionId];        $response = @socket_recv($connection, $data, 1024, 0);        /**         * False == No Data         * 0 == Widowed socket / Remote client closed connection         */        if($response === false || $response === 0)        {            $this->removeSocket($connectionId);            return '';        }        $response = $data;        if((ord(substr($response, -1)) == 26 || ord(substr($response, -1)) == 0 || is_null($response)) === false)        {            while($socketStatus = @socket_recv($connection, $data, 1024, 0))            {                /**                 * false =  no data                 */                if($socketStatus === false)                    break;                /**                 * 0 = widowed socket -> remote client closed connection;                 */                if($socketStatus === 0)                {                    $this->removeSocket($connectionId);                    break;                }                if(!is_null($data))                    $response .= $data;                if(ord(substr($data, -1)) == 26 || ord(substr($data, -1)) == 0 || is_null($data))                    break;            }        }        $response = substr($response, 0, -1);        return $response;    }    /**     * Reads information from the facelink service     * @return string Information from the facelink service     */    protected function readFacelinkSocket()    {        $response = ;        while(false !== @socket_recv($this->facelinkSocket, $data, 1024, 0))        {            if(!is_null($data))                $response .= $data;            if(ord(substr($data, -1)) == 26 || ord(substr($data, -1)) == 0 || is_null($data))            {                break;            }        }        return $response;    }    /**     * Main loop, starts the proxy     * @return void     */    public function run ()    {        /*         * Client connections' pool         */        $this->clients = array();        while (true)         {            if(!$this->shutdown)            {                $conn = @socket_accept($this->listenSocket);                if (is_resource($conn))                 {                    $conn_id = (integer) $conn;                    $this->clients[$conn_id] = $conn;                    $this->writeClientSocket($conn_id, 999 HELLO, false);                }            }            /**             * If we are shutting down and all the clients have finished their business, close up.             */            if($this->shutdown && !count($this->clients))            {                $this->quit();                break;            }            /**             * Create a copy of pool for socket_select()             */            $active = $this->clients;            /**             * Find active sockets             */            socket_select($active, $w = null, $e = null, null);            /**             * Handle every active client             */            foreach ($active as $conn)             {                $conn_id = (integer) $conn;                $this->proxyCommand($conn_id);            }            /**             * Sleep (a little), if we do the server starts to lock up             */            usleep(5000);        }        $this->shutdown();    }}
Review for a PHP socket proxy
php;object oriented
Why not place the magic numbers into your class as constants?class FacelinkProxyServer {   const USER_ID = 001 User Identification   // and so on}Then you can access them in the class as their name and outside as FacelinkProxyServer::USER_ID.