I always forget how to turn on picky error reporting in PHP; specifically, how to make PHP complain when you try to use unitialised variables. This is important if you are teaching PHP, as students typically make many spelling mistakes and/or character case mistakes.
As far as I can tell, to turn on strict error reporting which will catch these sorts of errors, set the following directive in php.ini:
error_reporting = E_ALL
You will need to restart PHP to get this to take effect.