Shortcodes have similarities with both action hooks and filter hooks, since their associated custom function is called when it is time to perform a task, just like an action hook, but they must return their output through a return value, just like a filter hook. In terms of external data, the function associated with a shortcode will receive data in the case of some types of codes, while it will only produce output in other cases.

When used in the text of a post or page, any shortcode surrounded by a pair of square brackets is identified by the WordPress engine, which then searches for functions registered for that specific code. If found, the associated function is called and the expected result is used to replace the original shortcode text in the item's content. Just like filter functions, shortcode functions must not output any text directly, since it would likely appear in an unexpected place in the page layout, as WordPress calls all shortcode-processing functions before displaying the body of an item.

For simple shortcodes, such as those in this recipe, the plugin functions associated with them must return information, but they do not receive any additional data through function parameters. That being said, they can rely on utility functions, such as get_the_ID, get_the_title, and other WordPress utility functions, to be able to produce the appropriate output. Other types of shortcodes seen in later recipes will have more context and configuration options. It is also possible for shortcodes to access stored options data, which will be covered in Chapter 3, User Settings and Administration Pages.