Others

Logging

log_message('error', 'Some variable did not contain a value.');

Level	Description
debug	Detailed debug information.
info	Interesting events in your application, like a user logging in, logging SQL queries, etc.
notice	Normal, but significant events in your application.
warning	Exceptional occurrences that are not errors, like the use of deprecated APIs, poor use of an API, or other undesirable things that are not necessarily wrong.
error	Runtime errors that do not require immediate action but should typically be logged and monitored.
critical	Critical conditions, like an application component not available, or an unexpected exception.
alert	Action must be taken immediately, like when an entire website is down, the database unavailable, etc.
emergency	The system is unusable.