Drupal - Run Custom Code When a Node's Workflow State Changes

Do you  need to execute some custom code when a node's workflow state changes in Drupal? Yes? Ok.

In your custom module, implement hook_workflow provided by the Workflow module and try something like this:

/**
 * Implementation of hook_workflow()
 */
function my_module_workflow($op, $old_sid, $sid, $node) {
  // We will load all the workflows and iterate over each so it is easier for
  // developers to make changes to any workflow.
  $workflows = workflow_get_all();
  if (isset($workflows)) {
    foreach ($workflows as $wid => $workflow) {
      switch ($op) {
        case "transition post":
          if ($workflow == "My Custom Workflow" || $wid == 123) {
            // Load up the workflow's states and determine
            // the old state and the current state.
            $worfklow_states = workflow_get_states($wid);
            $worfklow_old_state = $worfklow_states[$old_sid];
            $worfklow_current_state = $worfklow_states[$sid];
            drupal_set_message("$node->title had its workflow changed from $worfklow_old_state to $worfklow_current_state");
          }
          break;
      }
    }
  }
}

There are a few different $op values that are fed into your hook. So depending on what you are trying to do, you may want to use an $op value that is different from "transition post".