Can I leave out `if ( ! defined …)` when defining plugin constants?

By doing the !defined() check you allow earlier code to define that constant before you. This could be useful, for example, if a user is allowed to define its own values for some constants in a file like wp-config.php.

As for code readability, you may consider using the following one-liner, although you could put a regular if on a single line as well.

defined('XYZABC_PLUGIN_BASENAME') or define('XYZABC_PLUGIN_BASENAME', 'your_value');

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)