foo.php

<?php

function test() {
  $page1 = new SiteTree();
  $page1->ID = mt_rand();
  $page1->ParentID = 0;
  $page1->write();
  $page1->publish("Stage", "Live");
  
  $page2 = new SiteTree();
  $page2->ID = mt_rand();
  $page2->ParentID = $page1->ID;
  $page2->write();
  $page2->publish("Stage", "Live");
  
  $page3 = new SiteTree();
  $page3->ID = mt_rand();
  $page3->ParentID = $page2->ID;
  $page3->write();
  $page3->publish("Stage", "Live");
  
  // Current Heirachechy
  // Page 1 -> Page 2 -> Page 3
  
  // Now lets re-arrange the state of staging to cause the loop
  $page3->ParentID = $page1->ID;
  $page3->write();
  
  // Note that we write page3 before page2, otherwise we would cause a 'normal' infinite loop
  $page2->ParentID = $page3->ID;
  $page2->write();
  
  // New Heirachy (of staging)
  // Page1 -> Page 3 -> Page 2

  Versioned::set_reading_mode('Live');
  echo "While in Live mode: \r\n";
  echo "Page 3 parent: " . $page3->ParentID . "\r\n";
  echo "Page 2 parent: " . $page2->ParentID . " \r\n";

  Versioned::set_reading_mode('Stage');
  echo "While in Stage mode: \r\n";
  echo "Page 3 parent: " . $page3->ParentID . "\r\n";
  echo "Page 2 parent: " . $page2->ParentID . " \r\n";

While in Live mode:
Page 3 parent: 676509572
Page 2 parent: 2043093416
While in Stage mode:
Page 3 parent: 676509572
Page 2 parent: 2043093416

Versioned::reading_stage("Live"); not Versioned::set_reading_mode('Live');