Super simple shortcode not working

Your problem is here:

$welcomePage = get_post( 5 ); 
echo $welcomePage -> post_content;

Shortcodes are processed on display by via a filter on the_content which is called by the function the_content(). You’ve bypassed that functionality and are echoing the raw post data.

You need to output your code using a proper Loop or explicitly process the shortcode.

echo do_shortcode($welcomePage -> post_content);

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