The register_activation_hook function is used to indicate to WordPress the name of the function that should be called when it activates the plugin. Unlike other hooks, this function requires the name of the main plugin code file to be sent as its first argument, along with the name of the associated function. To do this easily, we can leverage the PHP __FILE__ constant as the first argument, which will resolve to the filename.

When the callback function is called, we can use the Options API to create, update, or, delete settings in the options table of the site's MySQL database. In this specific example, we are using the add_option function to easily create an option called ch3io_ga_account_name with a default value of UA-0000000-0.

Before making a call to create the new option, the activation function checks whether the option is present in the WordPress options table using the get_option function. If the return value is false, indicating that the option was not found, a new default option can be created. Any other result would show that the plugin has been activated on the site previously and that options may have been changed from their default values. It is important to keep in mind when writing this code that plugins get deactivated and reactivated each time they are updated using the WordPress update tool, resulting in a call to their activation function. It is also possible that a user might have deactivated a plugin temporarily to debug site issues and brought it back at a later time, also resulting in the activation function being called.

Finally, it should be noted that it is possible to call the add_option function multiple times if more than one option is needed to implement a plugin's desired functionality. That being said, it is not necessary to verify the presence of all the options, as checking for a single one would indicate that they were all previously set.