For this, Elgg has a cron endpoint.

Cron is a unix tool which executes commands at a specific time of day (other operating systems have similar tools). This keys off a file called a crontab – an example is given file is included and called crontab.example.

The crontab calls simplified yet powerful cron endpoint – http://yoursite/pg/cron/PERIOD, where PERIOD is one of the following:

• reboot – Execute on system reboot
• minute – Execute every minute
• fiveminute – Execute every five minutes
• fifteenmin – Execute every fifteen minutes
• halfhour – Execute every half hour
• hourly – Execute once every hour
• daily – Execute every day
• weekly – Execute weekly
• monthly – Execute once a month
• yearly – Execute every year

When these endpoints are triggered by your crontab a plugin hook is triggered. To make use of this, register a plugin hook as follows:

register_plugin_hook('cron', PERIOD, 'my_cron_handler');

Where PERIOD is one of the key words listed above. Here is some sample code using Cron – in this case it is taken from the system log rotation module I added to SVN today.

<?php
/**
* Elgg log rotator.
*
* @package ElggLogRotate
* @license http://www.gnu.org/licenses/old-licenses/gpl-2.0.html GNU Public License version 2
* @author Curverider Ltd
* @copyright Curverider Ltd 2008
* @link http://elgg.com/
*/

/**
* Initialise the plugin.
*
*/
function logrotate_init()
{
$period = get_plugin_setting('period','logrotate');
switch ($period)
{
case 'weekly':
case 'monthly' :
case 'yearly' :
break;
default: $period = 'monthly';
}

// Register cron hook
register_plugin_hook('cron', $period, 'logrotate_cron');
}

/**
* Trigger the log rotation.
*
*/
function logrotate_cron($hook, $entity_type, $returnvalue, $params)
{
$resulttext = elgg_echo("logrotate:logrotated");
if (!archive_log())
$resulttext = elgg_echo("logrotate:lognotrotated");

return $returnvalue . $resulttext;
}

// Initialise plugin
register_elgg_event_handler('init','system','logrotate_init');
?>