Boilerplate meta box

meta-box.php

<?php
/**
 * Class.
 *
 * @package a-package
 */

/**
 * Class.
 */
class Prefix_Page_Meta_Box {
	/**
	 * Constructor.
	 */
	public function __construct() {
		add_action( 'add_meta_boxes', array( $this, 'register' ), 10, 1 );
		add_action( 'save_post', array( $this, 'save' ), 10, 2 );
	}

	/**
	 * Register
	 *
	 * @param WP_Post $post The current post.
	 * @return void
	 */
	public function register( $post ) {
		add_meta_box( 'prefix_page_meta_box', __( 'Page details', 'text-domain' ), array( $this, 'html' ), 'page' );
	}

	/**
	 * Save.
	 *
	 * @param int     $post_id The post id.
	 * @param WP_Post $post The post object.
	 * @return void
	 */
	public function save( $post_id, $post ) {
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
			return;
		}

		if ( ! isset( $_POST['prefix_page_meta_box_nonce'] ) || ! wp_verify_nonce( $_POST['prefix_page_meta_box_nonce'], basename( __FILE__ ) ) ) {
			return $post_id;
		}

		$post_type = get_post_type_object( $post->post_type );

		if ( ! current_user_can( $post_type->cap->edit_post, $post_id ) ) {
			return $post_id;
		}

		$meta_keys = array(
			'prefix_example_1' => FILTER_SANITIZE_STRING,
			'prefix_example_2' => FILTER_SANITIZE_STRING
		);

		foreach ( $meta_keys as $meta_key => $filter ) {
			if ( ! empty( $_POST[ $meta_key ] ) ) {
				if ( $filter === 'array' || $filter === null ) {
					update_post_meta( $post_id, '_' . $meta_key, $_POST[ $meta_key ] );
				} else {
					update_post_meta( $post_id, '_' . $meta_key, filter_input( INPUT_POST, $meta_key, $filter ) );
				}
			} else {
				delete_post_meta( $post_id, '_' . $meta_key );
			}
		}
	}

	/**
	 * HTML.
	 *
	 * @param WP_Post $post The WordPress post object.
	 * @return void
	 */
	public function html( $post ) {
		$example_1 = get_post_meta( $post->ID, '_prefix_example_1', true );
		$example_2 = get_post_meta( $post->ID, '_prefix_example_2', true );

		wp_nonce_field( basename( __FILE__ ), 'prefix_page_meta_box_nonce' );
		?>
		<p>
			<label for="prefix_example_1"><?php _e( 'Example 1', 'text-domain' ); ?></label>
            		<?php wp_editor( $example_1, 'prefix_example_1' ); ?>
		</p>
        	<p>
			<label for="prefix_example_2"><?php _e( 'Example 2', 'text-domain' ); ?></label>
            		<input type="text" id="prefix_example_2" name="prefix_example_2" value="<?php echo esc_attr( $example_2 ); ?>">
		</p>
		<?php
	}
}

new Prefix_Page_Meta_Box();