Test post meta fields in WordPress

post-meta-test.php

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: Post Meta Test
 * Description: Adds a simple post meta field below the title to the post editor screen.
 * Plugin URI:  http://marketpress.com/
 * Version:     2014.09.12
 * Author:      MarketPress, Thomas Scholz
 * Author URI:  http://marketpress.com/
 * Licence:     GPL 2
 * License URI: http://opensource.org/licenses/GPL-2.0
 */

namespace MarketPress\Test;

if ( 'POST' === $_SERVER[ 'REQUEST_METHOD' ] )
	add_action( 'save_post',             __NAMESPACE__ . '\save_field', 10, 2 );
else
	add_action( 'edit_form_after_title', __NAMESPACE__ . '\show_field' );

/**
 * @param \WP_Post $post
 * @return void
 */
function show_field( \WP_Post $post ) {
	$key      = get_key();
	$field_id = $key . '_id';
	$value    = get_post_meta( $post->ID, $key, TRUE );

	wp_nonce_field( get_nonce_action(), get_nonce_name() );

	?>
	<p>
		<label for="<?php print $field_id; ?>">
			Post Meta Test
			<?php
			print "<input type='text' id='$field_id' name='$key' value='$value' class='regular-text'>";
			?>
		</label>
	</p>
	<?php
}

/**
 * @param int      $post_id
 * @param \WP_Post $post
 * @return bool|int
 */
function save_field( $post_id, \WP_Post $post ) {

	$key = get_key();

	if ( ! is_valid_request( $post ) )
		return FALSE;

	if ( empty ( $_POST[ $key ] ) )
		return delete_post_meta( $post_id, $key );

	return update_post_meta( $post_id, $key, $_POST[ $key ] );
}

/**
 * @param \WP_Post $post
 * @return bool
 */
function is_valid_request( \WP_Post $post ) {

	if ( wp_is_post_autosave( $post ) )
		return FALSE;

	if ( wp_is_post_revision( $post ) )
		return FALSE;

	if ( empty ( $_POST[ get_nonce_name() ] ) )
		return FALSE;

	return wp_verify_nonce( $_POST[ get_nonce_name() ], get_nonce_action() );
}

/**
 * @return string
 */
function get_key() {
	return '_marketpress_meta_test';
}

/**
 * @return string
 */
function get_nonce_action() {
	return 'marketpress_nonce_action';
}

/**
 * @return string
 */
function get_nonce_name() {
	return 'marketpress_nonce_name';
}