Wordpress — Generate parent/child page group arrays

output-page-group-array-usage.php

$id = get_the_id();
$array = output_page_group_array($id);

// Output page group array
var_dump($array);

output-page-group-array.php

function output_page_group_array($input) {
    
    // Get target post object from $input
    $post = get_post($input);

    // Create empty array
    $output = array();

    // If current page is a child/sibling
    if ($post->post_parent) {
        // Get current page parent
        $parent = get_post($post->post_parent);
        // Add parent item ID to array
        $output_parent = array($parent->ID);
        // Fetch child/sibling items
        $output_children = get_pages( array( 
            'child_of' => $post->post_parent,
        ) );
        // Pluck IDs & mount into array
        $output_children = wp_list_pluck( $output_children, 'ID' );
        // Merge parent & child arrays
        $output = array_merge($output_parent, $output_children);
    } 

    // If current page is a parent
    else {
        // Add parent item ID to array
        $output_parent = array($post->ID);
        // Fetch child/sibling items
        $output_children = get_pages( array( 
            'child_of' => $post->ID,
        ) );
        // Pluck IDs & mount into array
        $output_children = wp_list_pluck( $output_children, 'ID' );
        // Merge parent & child arrays
        $output = array_merge($output_parent, $output_children);
    }

    // Return page group array
    return $output;

}