If you have ever wondered what a WordPress functions.php file is used for, this file, also known as the theme functions file, is used to modify or add to the default behaviour of WordPress using PHP code. 

This file is located in the theme folder of the active theme that your website is currently using. You can find it at the following path: 

yourwebsitename.com/public/wp-content/themes/your-current-theme-name/functions.php

You can access this file location through FTP once you connect to your website. The functions.php file functions similarly to a plugin, allowing you to add custom features and functionality to your website. 

For example, you can add relevant snippets of code to this file to add custom features such as widgets, and post types, and even specify the number of posts to display on your website. 

If you are using a child theme, it will have its own functions.php file. This file will not overwrite the parent theme’s functions.php file. Instead, the child theme functions.php file is loaded in addition to the parent’s functions.php file.

The customization possibilities are limitless, thanks to the ability to add custom PHP code to the functions.php file.