php定义了if / else简写中的常量[duplicate]

I've been working on a php course, and one of the exercises has us create a config.php file wherein we define database constants.

I know the standard way of doing this, which is:

define("NAME", "value");

However, this exercise has it written differently. It's in if/else shorthand. Now I know it's correct, because it works. But I don't understand WHY it works. Hopefully it's a simple answer for you more experienced devs:

defined('DB_SERVER') ? null : define('DB_SERVER', 'localhost');

The way I read it, it's checking to see if DB_SERVER is defined. If it's true, then it sets it to NULL ?

Why would it NULL out the value of that constant if it's already defined?

</div>

If it's defined, it runs the expression null, which is essentially a noop (does nothing). Otherwise, it actually runs define. You could write this as defined('DB_SERVER') ?: define('DB_SERVER', 'localhost') nowadays, but I personally think that it is confusing. I would have simply written it as:

if (!defined('DB_SERVER')) {
    define('DB_SERVER', 'localhost');
}

Here is the long-hand form:

if ( defined('DB_SERVER') )
{
    // Do nothing
}
else
{
    define('DB_SERVER', 'localhost');
}

In English:

If someone already defined 'DB_SERVER', leave it alone

Otherwise, define it with the value of 'localhost'