Where do I add my custom PHP code?
Inside the child theme
The best way to add your custom PHP code (small snippets) is in the
functions.php file of your child theme. The functions file behaves like a WordPress Plugin, adding features and functionality to a WordPress site through PHP code. You can use it to call native PHP functions, WordPress functions, or to define your own functions.
Do not under any circumstance add PHP code in the
functions.php file of the parent theme! To keep your additions when you update the theme, always use the
functions.php file of your child theme.
Alternatively, simply use a "functionality" plugin such as the Code Snippets plugin to manage your custom PHP snippets.