gistfile1.aw

// check if tag already exists
$tag = 'a tag to contain a list of cacheIds';
$cacheId = 'some key to cache';

if ( ! $keys = $this->_cache->load( $tag ) ) {
    $keys = array( $cacheId );
    $this->_cache->save( $keys, $tag );
}
else if ( ! in_array( $cacheId, $keys ) ) {
    $keys[] = $cacheId;
    $this->_cache->save( $keys, $tag );
}