Made page anchor hashes more relevant to the page content

This will help when adding support for new kinds of page content such as markdown as we won't be able to reference the same ID's as before thus they would break on every save.

@@ -132,14 +132,13 @@ class PageRepo
         $childNodes = $body->childNodes;
 
         // Ensure no duplicate ids are used
-        $lastId = false;
         $idArray = [];
 
         foreach ($childNodes as $index => $childNode) {
             /** @var \DOMElement $childNode */
             if (get_class($childNode) !== 'DOMElement') continue;
 
-            // Overwrite id if not a bookstack custom id
+            // Overwrite id if not a BookStack custom id
             if ($childNode->hasAttribute('id')) {
                 $id = $childNode->getAttribute('id');
                 if (strpos($id, 'bkmrk') === 0 && array_search($id, $idArray) === false) {
@@ -149,13 +148,18 @@ class PageRepo
             }
 
             // Create an unique id for the element
-            do {
-                $id = 'bkmrk-' . substr(uniqid(), -5);
-            } while ($id == $lastId);
-            $lastId = $id;
+            // Uses the content as a basis to ensure output is the same every time
+            // the same content is passed through.
+            $contentId = 'bkmrk-' . substr(strtolower(preg_replace('/\s+/', '-', trim($childNode->nodeValue))), 0, 20);
+            $newId = urlencode($contentId);
+            $loopIndex = 0;
+            while (in_array($newId, $idArray)) {
+                $newId = urlencode($contentId . '-' . $loopIndex);
+                $loopIndex++;
+            }
 
-            $childNode->setAttribute('id', $id);
-            $idArray[] = $id;
+            $childNode->setAttribute('id', $newId);
+            $idArray[] = $newId;
         }
 
         // Generate inner html as a string