How to replace default comments with custom HTML?

Note that with the following:

add_filter ('comments_template', 'fbcommentbox', 100);

we expect fbcommentbox() to return the path to the new comments template file. The default is the path of comments.php.

If you create the fbcommentbox.php file in your theme, then you could try:

add_filter( 'comments_template', 'fbcommentbox', 100);

function fbcommentbox( $theme_template )
{
    // Path to our new comment template file
    $new_theme_template = get_template_directory() . '/fbcommentbox.php';

    // Override if it exsits
    if( file_exists( $new_theme_template ) )
        $theme_template = $new_theme_template;

    return $theme_template;
}

If the file fbcommentbox.php doesn’t exists then the default one is loaded instead. For a child theme we would use get_stylesheet_directory().

Here’s a shorter version:

function fbcommentbox( $theme_template )
{
    return locate_template( 'fbcommentbox.php' );
}

where the locate_template() does all the hard work.

Update

But you mentioned you wanted to override wp_list_comments() instead.

Here’s a way to do that by changing the echo argument to false:

add_filter( 'wp_list_comments_args', function( $args )
{
    // Display fbcommentbox.php 
    get_template_part( 'fbcommentbox' );

    // Disable output of wp_list_comments()
    $args['echo'] = 0;

    return $args;
} );

and output instead the fbcommentbox.php template.

Update

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