virtualcron.php

<?php
// +----------------------------------------------------------------------+
// | virtualCron 0.1                                                      |
// +----------------------------------------------------------------------+
// | Date: 6 Mar 2007                                                     |
// +----------------------------------------------------------------------+
// | License: LGPL                                                        |
// +----------------------------------------------------------------------+
// | virtualCron is a PHP class, that simulates a cron job,               |
// | in order to execute scripts periodically                             |
// | without a real crontab command.                                      |
// +----------------------------------------------------------------------+
// | Author: Giorgos Tsiledakis <gt(at)corissia(dot)com>                  |
// +----------------------------------------------------------------------+
//==============================================================================================
// To simulate a cron job, we need an absolute time value to estimate the time required to perform some task.
// This class uses the generation or last modification time of a control file on the server to achieve that.
//
// Advantage: no need of crontab commands or solutions like php preprocessing
// Disadvantage: a user access is required to perform some task
//
// Example:
// $vcron=new virtualcron(10,"virtualcron.txt"); // "10" minutes after the last modification of "virtualcron.txt"
// if ($vcron->allowAction()) print "Hello World"; // some action will be allowed
//
// Within the delay period nothing will happen -> allowAction=false
// if the delay period has passed the first user, who runs the script, with activate some action -> allowAction=true.
// All other users will be again within the delay period and nothing will happen. 
// That's all
// Use this e.g. to parse RSS Feeds every 30 minutes, to generate your google sitemap once a week etc.
// Of course, prefer a real cron job if your provider allows you to use one
//==============================================================================================
class virtualcron
{

    var $controlFile = "virtualcron.txt"; // the default url of the control file
    var $minDelay = "1"; // the default delay period in minutes

//==============================================================================================
// PUBLIC [Constructor]
// param $minDelay: sets the delay period in minutes (optional)
// param $conrolFile: sets the control file url (optional). The generation or last modification time
// of this file is used to estimate the time required to be passed, in order to allow an action.
// If there is no control file the function will try to generate one.
//==============================================================================================
    function virtualcron($minDelay = false, $controlFile = false)
    {
        if ($minDelay) $this->minDelay = $minDelay;
        if ($controlFile) $this->controlFile = $controlFile;
        $this->lastExec = 0; // it will contain the UNIXTIME of the last action
        $this->nextExec = 0; // it will contain the UNIXTIME of the next action
        $this->secToExec = 0; // it will contain the time in seconds until of the next action
        if (file_exists($this->controlFile)) $this->check = true;
        else {
            $handle = fopen($this->controlFile, "w");
            if (!$handle) $this->check = false;
            else {
                if (!fwrite($handle, time())) $this->check = false;
                else {
                    fclose($handle);
                    $this->check = true;
                }
            }
        }
    }
//==============================================================================================
// PUBLIC allowAction() [boolean]
// checks if the current execution time is within the delay period. Example:
// $vcron=new virtualcron();
// if ($vcron->allowAction()) ...do something...
// That's all
//==============================================================================================
    function allowAction()
    {
        $now = time();
        if ($this->check) $FT = $this->getFileCreationTime($this->controlFile);
        if ($FT) {
            $nextExec = $FT + ($this->minDelay * 60) - $now;
            if ($nextExec < 0) {
                $handle = fopen($this->controlFile, "w");
                if (!$handle) return false;
                else {
                    if (!fwrite($handle, $now)) return false;
                    else {
                        fclose($handle);
                        $this->lastExec = $now;
                        $this->nextExec = $now + ($this->minDelay * 60);
                        $this->secToExec = $this->minDelay * 60;
                        return true;
                    }
                }
            } else {
                $this->lastExec = $FT;
                $this->nextExec = $FT + $nextExec;
                $this->secToExec = $nextExec;
                return false;
            }
        } else return false;
    }
//==============================================================================================
// PRIVATE getFileCreationTime()
// estimates the generation or last modification time of the control file (UNIXTIME)
//==============================================================================================
    function getFileCreationTime($filename)
    {
        if (function_exists("filemtime")) {
            $FT = filemtime($filename);
        } else {
            $FT = false;
        }
        return $FT;
    }

}