WordPress functions.php

In a WordPress theme, the file named functions.php contains custom programming functions for that theme.

Description

WordPress themes can provide custom functionality via functions.php.

Writing custom functions requires some knowledge of PHP programming, more than a beginning programmer is likely to know.

However, beginning programmers may wish to study existing functions.php code from particular themes, to learn how functions work.
It is often possible for a beginner to modify an existing function for some specific purpose

functions.php is optional. Not all themes have it.

functions.php is located in the root of a WordPress theme folder.