The functions.php file in a WordPress theme allows you to add custom PHP functions to your theme. These functions can be used to enhance or modify the default behavior of your theme or WordPress itself.

Here’s how you can add code to the functions.php file:

  1. Backup Your Website: Before making any changes to your theme files, it’s essential to backup your website. This way, if something goes wrong, you can easily revert to a previous version.
  2. Access functions.php:
    • Via WordPress Dashboard:
      • Go to Appearance > Theme Editor.
      • On the right side, you’ll see a list of theme files. Click on functions.php.
    • Via FTP:
      • Connect to your website using an FTP client like FileZilla.
      • Navigate to /wp-content/themes/your-theme-name/ and find the functions.php file.
  3. Add Your Code: Scroll to the bottom of the functions.php file. Before the closing ?> tag (if it exists), add your custom PHP code. For instance: 
    function my_custom_function() {
// Your custom code here
}
add_action('init', 'my_custom_function');
  4. Save Changes:
    • If you’re using the WordPress dashboard, click on the Update File button.
    • If you’re using FTP, save the changes in your text editor and then upload the modified functions.php file back to the server.
  5. Test Your Website: After saving the changes, visit your website to ensure that everything works as expected. If you encounter any errors, you can revert to the backup you made earlier.

Note: It’s a good practice to use a child theme when making changes to your theme’s files. This ensures that your customizations won’t be lost when you update the parent theme. If you’re making extensive customizations, consider creating a custom plugin instead of adding lots of code to functions.php. This makes your custom functionality more portable and modular.