无法存储memcache密钥

Question

I need memcache for store specific configuration keys. I've integrate memcache inside my Settings class, this class is instatiated each time that a request is performed. Infact, in my own framework, all requests are shunt from index.php. So:

index.php -> autoloader (new Settings, new Model, new Controller, etc...) -> logic

the Settings class is like this:

include 'config.php'; //contains the `$config` array

class Settings
{
    private $_config = array();
    private static $_memcache = null;

    function __construct()
    {
       self::$_memcache = new Memcache();
       self::$_memcache->connect('localhost', 11211); //take in the php doc example

      $this->_config = $config;

      foreach($config as $item => $value)
      {
         self::$_memcache->add($item, $value);
      }
   }

   public static function set($name, $value)
   {
      self::$_memcache->set($name, $value);
   }

   public static function get($name)
   {
      return self::$_memcache->get($name);
   }
}

How you can see I use the method add, in the doc I've read that return false when a key is already setted. The value to set in memcache are taken from my $config array that is included by config.php file.

I also have two method set that should set a specific key, and get that should return a key.

The problem's that each time that this script is executed I get true when I var_dump(self::$_memcache->add($item, $value));.

I use self for use static method of settings. What am I doing wrong?