gistfile1.txt

<?php

/**
 *   Performs custom validation on custom post type "Site"
 */
function custom_post_site_save($post_id, $post_data) {
	# If this is just a revision, don't do anything.
	if (wp_is_post_revision($post_id))
		return;

	if ($post_data['post_type'] == 'site') {
		# In this example, we will deny post titles with less than 5 characters
		if (strlen($post_data['post_title'] < 5)) {
			# Add a notification
			update_option('my_notifications', json_encode(array('error', 'Post title can\'t be less than 5 characters.')));
			# And redirect
			header('Location: '.get_edit_post_link($post_id, 'redirect'));
			exit;
		}
	}
}
add_action( 'pre_post_update', 'custom_post_site_save', 10, 2);

/**
 *   Shows custom notifications on wordpress admin panel
 */
function my_notification() {
	$notifications = get_option('my_notifications');

	if (!empty($notifications)) {
		$notifications = json_decode($notifications);
		#notifications[0] = (string) Type of notification: error, updated or update-nag
		#notifications[1] = (string) Message
		#notifications[2] = (boolean) is_dismissible?
		switch ($notifications[0]) {
			case 'error': # red
			case 'updated': # green
			case 'update-nag': # ?
				$class = $notifications[0];
				break;
			default:
				# Defaults to error just in case
				$class = 'error';
				break;
		}

		$is_dismissable = '';
		if (isset($notifications[2]) && $notifications[2] == true)
			$is_dismissable = 'is_dismissable';

		echo '<div class="'.$class.' notice '.$is_dismissable.'">';
		echo '<p>'.$notifications[1].'</p>';
		echo '</div>';

		# Let's reset the notification
		update_option('my_notifications', false);
	}
}
add_action( 'admin_notices', 'my_notification' );