Each filter "class" implements a single method - %filter()% - which is called whenever a brigade is ready to be processed (writing/push mode), or whenever the caller would like some buckets from its output brigade (reading/pull mode). Each instance of a filter has a pair of brigades to hold buckets either pending transformation or consumption by the next filter in the chain. typedef struct _php_stream_filter_ops { php_stream_filter_status_t (*filter)( php_stream *stream, php_stream_filter *thisfilter, php_stream_bucket_brigade *buckets_in, php_stream_bucket_brigade *buckets_out, size_t *bytes_consumed, int flags TSRMLS_DC); void (*dtor)(php_stream_filter *thisfilter TSRMLS_DC); const char *label; } php_stream_filter_ops;