What's the difference between using PHP define constant and PHP $_GLOBALS?

I read somewhere that defining constants with PHP's define function like so:

define('BASE_PATH','/var/www/html/example/');

is better and more secure than storing the same variable data inside a globals variable like so:

$_GLOBALS['BASE_PATH'] = '/var/www/html/example/';

Could somebody please explain the difference, which is better in which scenarios, and why?

I've just read here:

That the 'define' function can cause unwanted oddities.

a) What are the security implications of both?
b) How does PHP manage and store each of the variable's data in physical memory?