providing access to post_id or post inside functions.php

When you add an action via add_action(), the variables that are available in the callback function are determined by the specific hook you’re using. The load-post.php hook does not pass any values through to the callback function.

However, when adding meta boxes the correct hook to use is add_meta_boxes, and that hook provides 2 values to the callback function $post_type and $post.

So if you want to conditionally add a meta box based on the post’s page template you can use this $post variable with get_page_template_slug():

add_action( 'add_meta_boxes', 'prefix_post_metabox_setup', 10, 2 ); // The 2 here is required to get both arguments.

function prefix_post_metabox_setup( $post_type, $post ) {
    if ( 'my-template.php' == get_page_template_slug( $post ) ) {
        add_meta_box(); // etc.
    }
}

Just be aware that this means that your meta boxes won’t appear until after the page/post is saved with the new template. In the block editor (née Gutenberg) this might require a manual refresh.

Also, it’s best practice to prefix your functions with something unique to your project. post_metabox_setup is too generic a name.

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