Using Custom HTML To Output Anything

I use this code to add some things, all is good but when I test with Query Monitor plugin I get error message

function demo_mt() {
    $html = '...';
    return $html;
}

array(
    'name'  => __( 'demo', 'meta-box' ),
    'type' => 'custom_html',
    'callback' => demo_mt,
),
    

Use of undefined constant demo_mt – assumed ‘demo_mt’ (this will throw an Error in a future version of PHP)

Wordpress: 5.9.1
php: 7.4.25