class-placeholder-block.php

<?php
// Placeholder Classes
if ( ! class_exists( 'Placeholder_Block' ) ) {
	class Placeholder_Block {
		public $name;
		function __construct( $name ) {
			$this->name = $name;
			add_action( 'init', array( $this, 'onInit' ) );
		}

		function ourRenderCallback( $attributes, $content ) {
			ob_start();
			require get_theme_file_path( "/blocks/{$this->name}.php" );
			return ob_get_clean();
		}

		function onInit() {
			wp_register_script( $this->name, get_stylesheet_directory_uri() . "/blocks/{$this->name}.js", array( 'wp-blocks', 'wp-editor' ) );

			register_block_type( "blocktheme/{$this->name}", array(
				'editor_script' => $this->name,
				'render_callback' => array( $this, 'ourRenderCallback' )
			) );
		}
	}
}

new Placeholder_Block( 'header' );

functions.php

<?php 

include ( get_theme_file_path( '/includes/class-placeholder-block.php' ) );

header.js

wp.blocks.registerBlockType("gtrpsblocktheme/header", {
  title: "Global Trade Review Header",
  edit: function () {
    return wp.element.createElement(
      "div",
      { className: "gtr-placeholder-block" },
      "Global Trade Review Header Placeholder"
    );
  },
  save: function () {
    return null;
  },
});

header.php

<?php 

// Header code goes here.