Sean Coates has created a simple version of a popular bit of Perl functionality and shares the code in hi slatest post - the "pie" ability to perform a search and replace, line by line, through a file using a regular expression.
I've often found myself looking for a PHP equivalent. Not to do simple substitutions, of course, but complex ones. And since I'm most comfortable with PHP, and a I have a huge library of snippets that I can dig out to quell a problem that I may have solved years ago, I've been meaning to fill this void for a while.
So, one evening, he came home and hashed out this version of the script. He includes the code in the post...it works on the command line and pulls in a file (though STDIN) and loops through the buffered version to perform the regex replace.