I have a issue with the admin site of prestashop. I'm trying to activate the caching system but it doesn't save the setting.

When I add a server (I'm using an Amazon ElasticCache server) saves it, but when I select the enable option and click Save, it redirects me to "Back Office > Preferences > Performance" but with a blank page and the admin tabs visible.

I go back to those settings again and i see that the caching option is disabled.

Also, there is a warning:

"To use Memcached, you must install the Memcache PECL extension on your server. http://www.php.net/manual/en/memcache.installation.php"

, even if i already installed memcached via yum.

I tried also, by modifying the settings.inc.php file editing define('_PS_CACHE_ENABLED_', '0'); to:

define('_PS_CACHE_ENABLED_', '1'); 

But i get 500 Internal Server Error in every page, so i prefer to leave it as before.

Any ideas?

I'm using PrestaShop 1.4.6.2 with Nginx 1.0.11 and PHP-FPM 5.3.8 in a CentOS 5.7 system.