Andrew Curioso has a post to his blog looking at error handling in PHP 5.3, specifically in creating a "stack" that can provide more than one error handler at a time.

As you may know, set_error_handler can be used to set a custom error handler in PHP. It will catch any errors that happen in the script (with a few notable exceptions). If the function returns false then error handling resumes as normal; otherwise it is assumed that the custom handler took care of things. The problem is that you can only have one error handler active at one time. The purpose of this code is to provide a error handeling stack for PHP.

His example uses closures, so pre-5.3 installs won't be able to use it, but it's an interesting proof of concept to show how you could set four different error handlers on one execution. A code snippet is included to show how it all works.