These are functions dealing with error handling and logging. They
allow you to define your own error handling rules, as well as modify
the way the errors can be logged. This allows you to change and
enhance error reporting to suit your needs.
With the logging functions, you can send messages directly to other
machines, to an email (or email to pager gateway!), to system logs,
etc., so you can selectively log and monitor the most important parts
of your applications and websites.
The error reporting functions allow you to customize what level and
kind of error feedback is given, ranging from simple notices to customized
functions returned during errors.