Array to string conversion notices

/wp-content/plugins/w3-total-cache/lib/W3/Cache.php on line 52

It doesn’t seem to like this bit of code:

function &instance($engine, $config = array()) {
static $instances = array();

$instance_key = sprintf(‘%s_%s’, $engine, md5(implode(”, $config)));

if (!isset($instances[$instance_key])) {
switch ($engine) {
case W3TC_CACHE_MEMCACHED:
require_once W3TC_LIB_W3_DIR . ‘/Cache/Memcached.php’;
$instances[$instance_key] = & ne