Writing is push-based and works like this: %php_stream_write()% or %fwrite()% sends ~n~ bytes to be written to the stream. A bucket is created from the buffer. The bucket is pushed to the topmost filter on the write stack. We wind the handle on the filter; it may tranforms buckets, and pushes them onto the next filter. repeat the last step until we hit the the underlying stream. Each bucket is written to the underlying stream. Process repeats until one of the steps filters in the stack produces no output buckets.