create metabox to activate slider

Don’t bother with the checkbox and metabox, that’s limiting:

• You’re only able to have a single slider, and have to add code for every new slider to add a new checkbox
• Data gets stored in post meta as a custom field, but querying posts by their post meta is super expensive and slow

So I present, an alternative that’s much, much easier to implement, and far more flexible:

Use a non-hierarchical Slider Taxonomy

This gives you:

• free archives should you want to expand on your sliders ( optional ofcourse )
• A free UI
• Querying posts for the slider is now super super fast. This is faster than a checkbox in a metabox by orders of magnitude
• The queries scale up as traffic increases and post counts increase. Post meta queries don’t scale
• A free user interface in the form of a checkbox list ( so it’s technically a metabox with a checkbox, funny how that turns out! )
• A slider management UI, under posts you’ll get a page where you can see all your sliders, name them, add descriptions, see how many posts they have, delete/add
• Slider meta! Now you have slider terms, you can add term meta! A term meta for transition style? Transition speed? So many possibilities
• Posts can now appear in multiple sliders

For example this post appears in a slider named homepage, no prizes for where that slider might appear:

A post can even appear in multiple sliders!

How Do I setup The Taxonomy?

You would call register_taxonomy on the init action, like this:

add_action( 'init', 'create_morteza_slider_tax' );

function create_morteza_slider_tax() {
    register_taxonomy(
        'morteza_slider',
        'post',
        array(
            'label' => __( 'Sliders' ),
            'hierarchical' => false,
            'public' => false
        )
    );
}

How Do I fetch the posts for my slider?

You use the taxonomy name as a parameter in the post query:

$query = new WP_Query( [
    'morteza_slider' => 'homepage' // <-- this is all that's needed
] );

Then follow it up with a standard post loop:

// a standard post loop
if ( $query->have_posts() ) {
    echo 'start slider markup';

    while ( $query->have_posts() ) {
        $query->the_post();
        // TODO: output this post as a slide
    }
    wp_reset_postdata();
    echo 'end slider markup';
}

Making sure to swap out the various bits, e.g. the “start slider markup” gets replaced with whatever your slider library requires to work