3.1 Using functions.php

Unlike the style.css file, the functions.php file of a child theme does not
override (cascade) its counterpart from the parent. Instead, it is loaded in
addition to the parent’s functions.php. Specifically, right before the parent’s file.

The fact that a child theme’s functions.php file
is loaded first means that you can make the user functions of your theme
pluggable, i.e., replaceable by a child theme by declaring them
conditionally. Example:

In that way, a child theme can replace a PHP function of the parent by
simply declaring it beforehand.