Force download file from plugin (change output http headers)

Question

I'm developing a plugin and I want to add a menu entry that forces a file download (a CSV).

There are a few posts in this forum and over the internet with a similar solution, all of them are quite old and that solution seems not to work anymore.

This is my current approach (based on the mentioned posts). This does force a file download, but the file is actually a generated HTML with all WP admin menu and everything.

MyPlugin::__construct()

$capability = 'manage_options';
$menu_slug  = 'myplugin-data-export';
$function   = [$this, 'exportCSVdata'];
add_submenu_page(null, '', '', $capability, $menu_slug, $function);

MyPlugin::exportCSVdata()

public function exportCSVdata()
{
    header("Content-type: application/x-msdownload");
    header("Content-Disposition: attachment; filename=data.csv");
    header("Pragma: no-cache");
    header("Expires: 0");
    echo 'data';
    exit();
}

This function is executed when I enter the URL http://example.com/wp-admin/admin.php?page=myplugin-data-export

So, the question maybe is,

how can I create a function inside my plugin class that generates a naked page? With no header, includes and all the WP html stuff?

In some ohter posts I saw a proposal to use admin-post.php instead of admin.php but didn't work either.

EDIT Plugin is being run as follows:

if ( !class_exists( 'MyPlugin' ) ) {
    class MyPlugin
    {
        static $instance = false;

        public static function getInstance()
        {
            if ( !self::$instance )
                self::$instance = new self;
            return self::$instance;
        }

        public function __construct()
        {
            register_activation_hook( __FILE__, [$this, 'mypluginInstall']);
            add_action('admin_menu', [$this, 'adminMenuEntries']);
        }

        public function adminMenuEntries()
        {
            //... some stuff
            $capability = 'manage_options';
            $menu_slug  = 'myplugin-data-export';
            $function   = [$this, 'exportCSVdata'];
            add_submenu_page(null, '', '', $capability, $menu_slug, $function);
        }
    }
}

Answer 1

how can I create a function inside my plugin class that generates a naked page? With no header, includes and all the WP html stuff?

There are many ways to do that, e.g. by using the admin_post hook just as you could see in @fuxia's answer here (which does work in WordPress 5.8.1 for what you're trying to do), or the parse_request hook as you could see in my answer here (for outputting an image data) — the WordPress REST API also uses the same hook.

But you said:

I'm developing a plugin and I want to add a menu entry that forces a file download (a CSV).

So if it has to be using a standard admin page URL like http://example.com/wp-admin/admin.php?page=myplugin-data-export, then you could hook your exportCSVdata() function on the load-<page hook> hook, where in your case, the <page hook> value would be the hook name as returned by add_submenu_page():

$hook = add_submenu_page('<parent slug>, '', '<menu title>', $capability, $menu_slug, $function);
add_action( "load-$hook", [ $this, 'exportCSVdata' ] );

Then clicking the menu item would force the file download and the (CSV) file would also have a valid data (and no admin menu HTML displayed on the page).

But then, you could actually link the menu to http://example.com/wp-admin/admin-post.php?action=print.csv by passing an empty string ('') as the 6th parameter for add_submenu_page(), and set the menu slug to admin-post.php?action=print.csv:

add_submenu_page('<parent slug>, '', '<menu title>', $capability, 'admin-post.php?action=print.csv', '');

That way, I bet the admin_post hook would work as expected.

So for example, in your class constructor, you would add add_action( 'admin_post_print.csv', [ $this, 'exportCSVdata' ] );.