Really useful way to take stress to you server is sending to the browser proper header establishing when your content will expire and until when needs to be cached.
Imagine how many less request you will have since the browser have that info already cached!
How to do it in PHP, just put this code at the top of your code, before anything it’s output.
$expire=60*60*24*1;// seconds, minutes, hours, days
header('Pragma: public');
header('Cache-Control: maxage='.$expire);
header('Expires: ' . gmdate('D, d M Y H:i:s', time()+$expire) . ' GMT');
header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . ' GMT');
Just be aware of the time you put, if your site needs to have the latest fresh news it may be not be agood Idea to use more than few minutes expire.
This will be implemented int he new version of Open Classifieds ;), 1.7. coming soon….
Source: php.net