note 112293 modified in language.types.null by danbrown

Be careful using NULL together with namespaces. If a NULL constant is redefined in a namespace other than global, you will get unexpected results when comparing to NULL inside the namespace. Instead always use \NULL, \FALSE, and \TRUE when comparing. Otherwise it may lead to application failures and potential security issues where certain checks could be effectively disabled. A simple example to demonstrate the behavior: <?php namespace RedefinedConstants { // redefining global namespace constants has no effect define('NULL', 'I am not global NULL!'); define('TRUE', 'I am not global TRUE!'); define('FALSE', 'I am not global FALSE!'); // redefining local namespace constants will work define('RedefinedConstants\NULL', 'I am not NULL!', \TRUE); define('RedefinedConstants\FALSE', 'I am not FALSE!', \TRUE); define('RedefinedConstants\TRUE', 'I am not TRUE!', \TRUE); var_dump( NULL, \NULL, null, \null, Null, \Null, FALSE, \FALSE, false, \false, False, \False, TRUE, \TRUE, true, \true, True, \True ); } ?> --was-- Be careful using NULL together with namespaces. If a NULL constant is redefined in a namespace other than global, you will get unexpected results when comparing to NULL inside the namespace. Instead always use \NULL, \FALSE, and \TRUE when comparing. Otherwise it may lead to application failures and potential security issues where certain checks could be effectively disabled. A simple example to demonstrate the behavior: namespace RedefinedConstants { // redefining global namespace constants has no effect define('NULL', 'I am not global NULL!'); define('TRUE', 'I am not global TRUE!'); define('FALSE', 'I am not global FALSE!'); // redefining local namespace constants will work define('RedefinedConstants\NULL', 'I am not NULL!', \TRUE); define('RedefinedConstants\FALSE', 'I am not FALSE!', \TRUE); define('RedefinedConstants\TRUE', 'I am not TRUE!', \TRUE); var_dump( NULL, \NULL, null, \null, Null, \Null, FALSE, \FALSE, false, \false, False, \False, TRUE, \TRUE, true, \true, True, \True ); } http://php.net/manual/en/language.types.null.php