How do I turn off PHP Notices?

From the PHP documentation (error_reporting):

<?php// Turn off all error reportingerror_reporting(0);?>

Other interesting options for that function:

<?php// Report simple running errorserror_reporting(E_ERROR | E_WARNING | E_PARSE);// Reporting E_NOTICE can be good too (to report uninitialized// variables or catch variable name misspellings ...)error_reporting(E_ERROR | E_WARNING | E_PARSE | E_NOTICE);// Report all errors except E_NOTICE// This is the default value set in php.inierror_reporting(E_ALL & ~E_NOTICE);// For PHP < 5.3 use: E_ALL ^ E_NOTICE// Report all PHP errors (see changelog)error_reporting(E_ALL);// Report all PHP errorserror_reporting(-1);// Same as error_reporting(E_ALL);ini_set('error_reporting', E_ALL);?>

You can set display_errors to 0 or use the error_reporting() function.

However, notices are annoying (I can partly sympathize) but they serve a purpose. You shouldn't be defining a constant twice, the second time won't work and the constant will remain unchanged!

For the command line php, set

error_reporting = E_ALL & ~E_NOTICE

in /etc/php5/cli/php.ini

command php execution then ommits the notices.