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Why 'defined() || define()' syntax in defining a constant

This piece of code is created by the zf tool that Zend Framework provides.

defined('APPLICATION_PATH') || define('APPLICATION_PATH', realpath(dirname(__FILE__) . '/../application'));

My questions is :
What is the purpose of this line of code? There are no conditional statements such as if, switch. Does it imply a conditional statement automatically?

Here is how i understand it:
if APPLICATION_PATH is defined, leave it alone else set it to : realpath(dirname(__FILE__) . '/../application').

If my assumption is true this is a really confusing syntax.

Any help will be appreciated.

Answer 1

Your assumption is correct. This is just a short hand way of saying

if (!defined('APPLICATION_PATH')) {
  define('APPLICATION_PATH', '...');
}

You can easily test this:

define("foo", "bar");
defined("foo") || define("foo", "baz");
var_dump(foo);

Output is bar.

//define("foo", "bar");
defined("foo") || define("foo", "baz");
var_dump(foo);

Output is baz.