Set up the ads.txt file

/**
 * Register the rewrite rule for /ads.txt request.
 */
function my_theme_adstxt_rewrite() {
    add_rewrite_rule( '^ads\.txt$', 'index.php?my_theme_adstxt=true', 'top' );
}
add_action( 'init', 'my_theme_adstxt_rewrite', 10 );
 
/**
 * Filter the list of public query vars in order to allow the WP::parse_request
 * to register the query variable.
 *
 * @param array $public_query_vars The array of public query variables.
 *
 * @return array
 */
function my_theme_adstxt_query_var( $public_query_vars ) {
    $public_query_vars[] = 'my_theme_adstxt';
    return $public_query_vars;
}
add_filter( 'query_vars', 'my_theme_adstxt_query_var', 10, 1 );
 
/**
 * Hook the parse_request action and serve the ads.txt when custom query variable is set to 'true'.
 *
 * @param WP $wp Current WordPress environment instance
 */
function my_theme_adstxt_request( $wp ) {
    if ( isset( $wp->query_vars['my_theme_adstxt'] ) && 'true' === $wp->query_vars['my_theme_adstxt'] ) {
        /*
         * Set proper content-type per specification in
         * https://iabtechlab.com/wp-content/uploads/2017/09/IABOpenRTB_Ads.txt_Public_Spec_V1-0-1.pdf :
         *
         * The HTTP Content-type should be ‘text/plain’, and all other Content-types should be treated
         * as an error and the content ignored.
         */
        header( 'Content-Type: text/plain' );
 
        // The code expects an existing ads.txt file in the root of your active theme.
        echo file_get_contents( get_stylesheet_directory() . '/ads.txt' );
        exit;
    }
}
add_action( 'parse_request', 'my_theme_adstxt_request', 10, 1 );