Moving from Zend Memcache

[eluser]RaZoR LeGaCy[/eluser]
Looking to move from using Zend Framework Memcache but need help with class.

I have models that use pre-existing code and need to stay that way. The old code uses Cache::init($lifetime of cache); so I need to keep it this way. Problem is, I cannot get the $_cache variable and the $lifetime variable into the save function.

I try $this-> and it says "Using $this when not in object context" and when using $lifetime, is says "Undefined variable: lifetime"; the same for $_cache

Code:

class Cache
{
   public $_cache;
   public $lifetime;

    //static protected $_cache;

    function init($lifetime=3600) {
      $_cache = new Memcache;
      $_cache->addServer('127.0.0.1', 11211);
   }

    static function getInstance() {
        return self::$_cache;
    }
    
    static function save($key, $value) {
       return $_cache->set($key, $value, 0, time()+$lifetime);
    }
}

What is wrong here?