Wordpress: send email to subscriber when the post is published.

gistfile1.txt

//Just paste the following code on your functions.php file

function email_members($post_ID)  {
    global $wpdb;
    $usersarray = $wpdb->get_results("SELECT user_email FROM $wpdb->users;");    
    $users = implode(",", $usersarray);
    mail($users, SUBJECT, CONTENT);
    return $post_ID;
}

add_action('publish_post', 'email_members');