struct _php_stream_filter {
            php_stream_filter_ops *fops;
            void *abstract; /* for use by filter implementation */
            php_stream_filter *next;
            php_stream_filter *prev;
            int is_persistent;

            /* link into stream and chain */
            php_stream_filter_chain *chain;

            /* buffered buckets */
            php_stream_bucket_brigade buffer;
        } php_stream_filter;