Simple error handling class can enable PHP execution error logging setting the file that logs error messages. It provides an error handler function that outputs a stack back-trace with the functions, arguments, file and lines of where an error occurs, as well the values of super-global variables $_GET, $_POST and $_FILES, and also the IP address of the current user accessing the site.

The output of error messages can also be enabled or disabled and the error reporting level can be changed.

When used together with my MySQL class (or any other mysql class that calls trigger_error when there was mysql_error) it can effectively log all mysql errors with the complete stacktrace what you can inspect from the logfile later on.
Plus you can identify sql injections when someone's queries keep failing with error messages.

You can also specify error codes that exit the script execution plus your own callback function that will be called before exiting (good to display some nice error page to users before exiting script execution).

Requirements:
PHP 5.0 or higher