If you have access to your php.ini, then Bj?rn answer is the way to go.
However, if you don't, or if you want to change a particular script / project error level, do this at the beginning of your code:
ini_set('display_errors', 1);
// Enable error reporting for NOTICES
error_reporting(E_NOTICE);
You can see which levels are available for error_reporting here: http://us2.php.net/manual/en/function.error-reporting.php.
It's always a good practice not to show any errors on production environments, but logging any weird behaviors and sending by mail to the administrator. NOTICES should only be enabled on development environments.
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