gistfile1.php

<?php

class AF_Metaboxes {

  public function __construct() {
		$this->hooks();
	}

	public function hooks() {
		add_action( 'add_meta_boxes', array( $this, 'metaboxes' ) );
		add_action( 'save_post', array( $this, 'save' ) );
	}

	public function metaboxes() {
		add_meta_box( 'extra-fields', 'New Post Meta', array( $this, 'extra_fields_metaboxes' ), 'page', 'side' );
	}

	public function	extra_fields_metaboxes() {
		$nonce = wp_create_nonce( 'af_meta_nonce' );
		$related = get_post_meta( $post_id, 'af_meta', true );
		?>
		<input type="hidden" name="_meta_nonce" value="<?php echo $nonce ?>" />
		<div class="inside">
			<label class="screen-reader-text">My Meta</label>
			<p><input type="text" name="af_meta" value="<?php echo esc_attr( $af_meta ) ?>" /></p>
			<p class="howto">Some help text</p>
		</div>
		<?php
	}

	public function save( $post_id )
	{
		if( !wp_verify_nonce( $_POST['_meta_nonce'], 'af_meta_nonce' ) )
			return false;

		update_post_meta( $post_id, 'af_meta_nonce', $_POST['af_meta'] );
	}
}