Transition Post status not working

Question

I have a plugin which uses the transition_post_status hook, to fire a custom function on post update. But I don't know why its not working. Here is the code

 add_action('transition_post_status', 'pmc_update_notification',10,3);

function pmc_update_notification($new_status, $old_status, $post) {
echo 'function checkpoint 1';
$options = get_option('pmc_setting');
if($options['snpi'] != false && is_single()){
   if ( $old_status == 'publish' && $new_status == 'publish' ) {
  echo 'function checkpoint 2';
$post_title = get_the_title($post);
$post_url = get_permalink($post);
$message = __('Post updated','pmc').":\n";
$message .= $post_title . ": " . $post_url;

// Send notification
pmc_send($message);
  }
 }
}

Answer 1

You have to called this function in your header.php (your wp theme) :

<?php wp_head(); ?>

And this one into your Footer.php :

<?php wp_footer(); ?>

Because you are adding action to your Wordpress, so you have to call that function in your theme.

Answer 2

Found the problem!

It was the is_single() line. It's not working because its outside the loop.