Create WordPress metaboxes OOP style

wp-metabox-oop-example.php

<?php
/**
 * Plugin Name:       CHB Meta Box test
 * Plugin URI:        http://www.chrispian.com
 * Description:       Just testing out OOP Meta Boxes.
 * Version:           0.0.1
 * Requires at least: 5.3
 * Requires PHP:      7.2
 * Author:            Chrispian H. Burks
 * Author URI:        http://chrispian.com
 * License:           GPL v2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 * Text Domain:       chb-meta-box-test
 * Domain Path:       /languages
 */

namespace CHB;

/*
Reference: https://developer.wordpress.org/plugins/metadata/custom-meta-boxes/
*/

class Meta_Box
{

  public static function add() {
		$screens = [ 'post', 'page' ];
		foreach ($screens as $screen) {
			add_meta_box(
				'test_box_id',          // Unique ID
				'Test Meta Box',        // Box title
				[self::class, 'html'],  // Content callback, must be of type callable
				$screen,                // Post type
				'side'                  // Varies based on screen. Ex: Screen = post contexts: 'normal', 'side', and 'advanced'
			);
		}
	}

	public static function save( $post_id ) {
		if ( array_key_exists( 'chb_test_field', $_POST ) ) {
			update_post_meta(
				$post_id,
				'_chb_test_meta_key',
				$_POST['chb_test_field']
			);
		}
	}


	public static function html( $post ) {
		$value = get_post_meta( $post->ID, '_chb_test_meta_key', true );
		?>
    <label for="chb_test_field"><strong>Test Option</strong></label>
		<select name="chb_test_field" id="chb_test_field" class="postbox">
			<option value="">Select</option>
			<option value="Option 1" <?php selected($value, 'Option 1'); ?>>Option 1</option>
			<option value="Option 2" <?php selected($value, 'Option 2'); ?>>Option 2</option>
		</select>
		<?php
	}
}

add_action('add_meta_boxes', ['CHB\Meta_Box', 'add']);
add_action('save_post', ['CHB\Test_Meta_Box', 'save']);