The is_admin function is used to quickly tell if the page currently being rendered is an administration page. If it is, our plugin code uses the standard PHP include function to load and execute the contents of a separate file. In this case, the file is a second PHP file located in the plugin directory. To be flexible with regards to the location of the plugin files, we build a path to the file containing the administration functions using the WordPress plugin_dir_path function.

While the benefit of placing so little code in a separate file is minimal, this technique has a larger impact on performance when dealing with larger administration panels. In addition to not having to register an action hook on every page load, the PHP interpreter does not have to make sure that the syntax for the entire contents of that second file is valid when rendering front-facing pages.