wp_using_themes example

<?php /** * Loads the correct template based on the visitor's url * * @package WordPress */
if ( wp_using_themes() ) {
    /** * Fires before determining which template to load. * * @since 1.5.0 */
    do_action( 'template_redirect' );
}

/** * Filters whether to allow 'HEAD' requests to generate content. * * Provides a significant performance bump by exiting before the page * content loads for 'HEAD' requests. See #14348. * * @since 3.5.0 * * @param bool $exit Whether to exit without generating any content for 'HEAD' requests. Default true. */