Update Posts transformer to extract images by splitting the DOM tree

lib/Util/DOM.php

@@ -0,0 +1,40 @@
+<?php
+namespace MailPoet\Util;
+use pQuery\DomNode;
+
+class DOM {
+
+  /**
+   * Splits a DOM tree around the cut element, bringing it up to bound
+   * ancestor and splitting left and right siblings into subtrees along
+   * the way, retaining order and nesting level.
+   */
+  static function splitOn(DomNode $bound, DomNode $cut_element) {
+    $ignore_text_and_comment_nodes = false;
+    for($parent = $cut_element->parent; $bound != $parent; $parent = $grandparent) {
+      // Clone parent node without children, but with attributes
+      $parent->after($parent->toString());
+      $right = $parent->getNextSibling($ignore_text_and_comment_nodes);
+      $right->clear();
+
+      while($sibling = $cut_element->getNextSibling($ignore_text_and_comment_nodes)) {
+        $sibling->move($right);
+      }
+
+      // Reattach cut_element and right siblings to grandparent
+      $grandparent = $parent->parent;
+      $index_after_parent = $parent->index() + 1;
+      $right->move($grandparent, $index_after_parent);
+      $index_after_parent = $parent->index() + 1;
+      $cut_element->move($grandparent, $index_after_parent);
+    }
+  }
+
+  static function findTopAncestor(DomNode $item) {
+    while($item->parent->parent !== null) {
+      $item = $item->parent;
+    }
+    return $item;
+  }
+
+}