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;