When are infinite loops are useful in PHP?

Question

While reading through the great online PHP tutorials of Paul Hudson he said

Perhaps surprisingly, infinite loops can sometimes be helpful in your scripts. As infinite loops never terminate without outside influence, the most popular way to use them is to break out of the loop and/or exit the script entirely from within the loop whenever a condition is matched. You can also rely on user input to terminate the loop - for example, if you are writing a program to accept people typing in data for as long as they want, it just would not work to have the script loop 30,000 times or even 300,000,000 times. Instead, the code should loop forever, constantly accepting user input until the user ends the program by pressing Ctrl-C.

Would you please give me a simple running example of how to use infinite loops in PHP ?

Answer 1

Monitoring applications

If you have a background process that monitors the state of your servers and sends an email if trouble occurs. It would have an infinite loop to repeatably check the servers (with some pause between iterations.)

Server listening for Clients

If you have a server script that listens to a socket for connections, it will loop infinitely, blocking while waiting for new clients to connect.

Video Games

Games usually have a "game loop" that runs once a frame, indefinitely.

Or... anything else that needs to keep running in the background with periodic checks.

Answer 2

If you implemented a socket server (taken from: http://devzone.zend.com/article/1086 ):

    #!/usr/local/bin/php –q

<?php
// Set time limit to indefinite execution
set_time_limit (0);

// Set the ip and port we will listen on
$address = '192.168.0.100';
$port = 9000;
$max_clients = 10;

// Array that will hold client information
$clients = Array();

// Create a TCP Stream socket
$sock = socket_create(AF_INET, SOCK_STREAM, 0);
// Bind the socket to an address/port
socket_bind($sock, $address, $port) or die('Could not bind to address');
// Start listening for connections
socket_listen($sock);

// Loop continuously
while (true) {
    // Setup clients listen socket for reading
    $read[0] = $sock;
    for ($i = 0; $i < $max_clients; $i++)
    {
        if ($client[$i]['sock']  != null)
            $read[$i + 1] = $client[$i]['sock'] ;
    }
    // Set up a blocking call to socket_select()
    $ready = socket_select($read,null,null,null);
    /* if a new connection is being made add it to the client array */
    if (in_array($sock, $read)) {
        for ($i = 0; $i < $max_clients; $i++)
        {
            if ($client[$i]['sock'] == null) {
                $client[$i]['sock'] = socket_accept($sock);
                break;
            }
            elseif ($i == $max_clients - 1)
                print ("too many clients")
        }
        if (--$ready <= 0)
            continue;
    } // end if in_array

    // If a client is trying to write - handle it now
    for ($i = 0; $i < $max_clients; $i++) // for each client
    {
        if (in_array($client[$i]['sock'] , $read))
        {
            $input = socket_read($client[$i]['sock'] , 1024);
            if ($input == null) {
                // Zero length string meaning disconnected
                unset($client[$i]);
            }
            $n = trim($input);
            if ($input == 'exit') {
                // requested disconnect
                socket_close($client[$i]['sock']);
            } elseif ($input) {
                // strip white spaces and write back to user
                $output = ereg_replace("[ \t\n\r]","",$input).chr(0);
                socket_write($client[$i]['sock'],$output);
            }
        } else {
            // Close the socket
            socket_close($client[$i]['sock']);
            unset($client[$i]);
        }
    }
} // end while
// Close the master sockets
socket_close($sock);
?> 

Answer 3

Maybe it useful when you write a command line PHP application? Because when PHP-scripts runs by web-server (Apache or any other) they lifetime is limited by 30 seconds by default (or you can manually change this limit at the configuration file).