saving rss feed into apc cache-Collection of common programming errors

i have a rss reader, i read rss-es using this line:

$content = @simplexml_load_file($feed); //$feed is the url to an rss

because this has gotten slow (i read many rss-es before serving it to the page), i want to cache it to the memory, so i changed it to the following:

if(apc_exists('feed'.$feed))
{
     $content = apc_fetch('feed'.$feed);
}  
else
{
    $content = @simplexml_load_file($feed);

        apc_store('feed'.$feed, $content, 2700+rand(0,1800));           
}

the problem now is that i get an error: Uncaught exception ‘Exception’ with message ‘Serialization of ‘SimpleXMLElement’ is not allowed’

how do i achieve this? I want to have the cache in the memory, i do not want file-based cache.

EDIT: Solved with:

if(apc_exists('feed'.$feed))
        {
            $a = apc_fetch('feed'.$feed);
        }  
        else
        {
            $a = file_get_contents($feed);
            apc_store('feed'.$feed, $a, 2700+rand(0,1800));         
        }
        $content = @simplexml_load_string($a);