WordPress Plugin: Pick the specific users to send an email to notifying them about a new post.

send-post-by-email.php

<?php

/*
Plugin Name: Specific User Notification
Plugin URI: https://gist.github.com/1306710
Description: Pick the specific users to send an email to notifying them about a new post.
Version: 0.2
Author: Nate Jacobs
Author URI: http://natejacobs.org
License: GPLv2 or later
*/

class UserPostNotificationByEmail
{	
	/** 
	 *	Construct
	 *
	 *	Hook into the necessary actions needed to add the meta box and send email.
	 *
	 *	@author		Nate Jacobs
	 *	@since		0.1
	 */
	public function __construct()
	{
		add_action( 'add_meta_boxes', array( __CLASS__, 'add_user_list_meta' ) );
		add_action( 'publish_post', array( __CLASS__, 'send_email_users' ), 10, 2 );
		add_action( 'admin_notices', array( __CLASS__, 'post_admin_notices') );
	}
	
	/** 
	 *	Add User List Meta Box
	 *
	 *	Registers the custom post meta box to display on all post edit pages.
	 *
	 *	@author		Nate Jacobs
	 *	@since		0.1
	 */
	public function add_user_list_meta()
	{
		add_meta_box( 'user_list', 'Users', array( __CLASS__, 'display_users_meta_box' ), 'post', 'side' );
		add_meta_box( 'email_list', 'Enter email addresses to send notice of the post to. Separate addresses with a comma.', array( __CLASS__, 'display_email_meta_box' ), 'post', 'normal' );
	}
	
	/** 
	 *	Display Users Meta Box
	 *
	 *	Creates the actual post meta box. It lists all users of the site with a
	 *	a checkbox next to their display name.
	 *
	 *	@author		Nate Jacobs
	 *	@since		0.1
	 */
	public function display_users_meta_box( $post )
	{
		$users = get_users();
		// loop through each user
		foreach ( $users as $user )
		{
			echo '<input type="checkbox" name="users_email['.$user->ID.']" value='.$user->user_email.'> '.$user->display_name;
			echo '<br>';
		}
	}
	
	/** 
	 *	Display Email Meta Box
	 *	
	 *	Creates the text field to enter email addresses manually
	 *
	 *	@author		Nate Jacobs
	 *	@since		0.2
	 */
	public function display_email_meta_box()
	{
		echo '<input type="text" name="additional_email" size="120">';
	}
	/** 
	 *	Post Redirect
	 *
	 *	Adds a query string to the url to indicate one of the email addresses was invalid
	 *
	 *	@author		Nate Jacobs
	 *	@since		1.0
	 */
	public function post_redirect( $location )
	{
		remove_filter( 'redirect_post_location', array( __CLASS__, 'post_redirect' ), '99' );
		return add_query_arg( 'bad_email', 1, $location );
	}
	/** 
	 *	Post Admin Notices
	 *
	 *	Adds an error message if one of the email addresses was formatted inforrectly
	 *
	 *	@author		Nate Jacobs
	 *	@since		0.2
	 */
	public function post_admin_notices()
	{
		if ( !isset( $_GET['bad_email'] ) ) 
			return;
		switch ( absint( $_GET['bad_email'] ) )
		{
			case 1:
      			$message = 'One of the email addresses you entered was invalid. Please check them for errors.';
		    	break;
			default:
				$message = 'Unexpected error';
		}
		echo '<div id="notice" class="error"><p>' . $message . '</p></div>';
	}
	
	/** 
	 *	Send Email to Users
	 *
	 *	If the post is being published for the first time send an email
	 *	to the users specified in the edit post screen.
	 *
	 *	Email Format
	 *	Hello [display name].
	 *	Just wanted to let you know there is a new post on [blog name].
	 *	Here is the link [permalink]
	 *
	 *	@author		Nate Jacobs
	 *	@since		0.1
	 *	@update		0.2		Added additional wp_mail for manually entered email addresses
	 */
	public function send_email_users( $post_id )
	{
		if( $_POST['post_status'] == 'publish' ) //&& ( $_POST['original_post_status'] != 'publish' ) )
		{
			if ( isset( $_POST['users_email'] ) )
			{	
				foreach( $_POST['users_email'] as $user )
				{
					$user_value = get_user_by( 'email', $user );
					$message = sprintf( __( 'Hello %s.' ), $user_value->display_name ). "\r\n\r\n";
					$message .= sprintf( __( 'Just wanted to let you know there is a new post on %s.' ), get_option( 'blogname' ) ). "\r\n\r\n";
					$message .= sprintf( __( 'Here is the link %s' ), get_permalink( $post_id ) );
					
					wp_mail( $user, sprintf( __( '[%s] A New Post' ), get_option( 'blogname' ) ), $message );
				}
			}
			
			if ( isset( $_POST['additional_email'] ) )
			{
				$email_addy = explode( ',', $_POST['additional_email'] );
				foreach( $email_addy as $email )
				{
					// check if email addresses are valid
					if ( !is_email( $email ) )
					{
						// if not, set a page redirect
						add_filter( 'redirect_post_location', array( __CLASS__, 'post_redirect' ), '99' );
					}
					else
					{
						$message = sprintf( __( 'Hello %s.' ), $email ). "\r\n\r\n";
						$message .= sprintf( __( 'Just wanted to let you know there is a new post on %s.' ), get_option( 'blogname' ) ). "\r\n\r\n";
						$message .= sprintf( __( 'Here is the link %s' ), get_permalink( $post_id ) );
						
						wp_mail( $email, sprintf( __( '[%s] A New Post' ), get_option( 'blogname' ) ), $message );
					}
				}
			}
		}
	}
}

new UserPostNotificationByEmail;