In a new post to his blog Sameer takes a look at streams and filters in PHP applications, specifically how to set up a custom filter to attach to any stream.

In this post we will see how to create a custom stream filter. Streams, first introduced in PHP 4.3, provide an abstration layer for file access. A number of different resources besides files - like network connections, compression protocols etc. can be regarded as "streams" of data which can be serially read and written to.

Streams are the method "behind the scenes" for several of the PHP functions you already use (like fsockopen and file_get_contents). His example shows you how you can manually create a stream and attach a filter to it to replace URLs in the contents of the stream with the "[---URL---]" string and push it back to the standard output.