Have you ever had a WordPress site crash during development or even when live? Often you will see errors of the form:
Notice: Undefined variable: user_id in
on line 166
Once you see this error you know something is wrong. However you don't want users to see this error because they are *ugly*. In addition, you want to know if the site is having problems when you are not looking at it.
To log these errors in WordPress, add the following to wp-config.php:
These commands will create a log file in /wp-content/debug.log and all the errors will be written there. You can examine this file for useful error messages generated by any user of the site.