only use memcached plugin if memcached responds

drupal6_memcached_single.php

/**
 * Use Memcached (if its on):
 *
 * Enable these configurations if a Memcached server is running on the
 * server. This will move the database load from MySQL to Memcached,
 * increasing the site speed and lowering the database lag.
 *
 * cache_inc    = replaces the Drupal caching mechanism
 * session_inc  = replaces the Drupal mysqldb Sessions handler
 * memcache_key_prefix should be set to a short unique string in multi Drupal environment
 *
*/
if (@memcache_connect('127.0.0.1',11211) !== false) {
    $conf['cache_inc']   = './sites/all/modules/memcache/memcache.inc';
    $conf['session_inc'] = './sites/all/modules/memcache/memcache-session.inc';
    $conf['memcache_key_prefix']= 'live';
}