WordPress: When a post is published, trigger an event

transaction.php

<?php
/* 
 * Add this function somewhere in functions.php 
 * You will see there is nothing is magic in WordPress
**/

function on_all_status_transitions( $new_status, $old_status, $post ) {
	if ('publish' == $new_status && 'publish' != $old_status && 'trash' != $old_status) {
	
	// Your code comes here
	
  }
}
add_action(  'transition_post_status',  'on_all_status_transitions', 10, 3 );