mysql - php defined constant in if/else shorthand -

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this question has answer here:

i've been working on php course, , 1 of exercises has create config.php file wherein define database constants.

i know standard way of doing this, is:

define("name", "value");

however, exercise has written differently. it's in if/else shorthand. know it's correct, because works. don't understand why works. it's simple answer more experienced devs:

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

the way read it, it's checking see if db_server defined. if it's true, sets null ?

why null out value of constant if it's defined?

if it's defined, runs expression null, noop (does nothing). otherwise, runs define. write defined('db_server') ?: define('db_server', 'localhost') nowadays, think confusing. have written as:

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

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