diff --git a/include/webdav_cache_validation.inc.php b/include/webdav_cache_validation.inc.php
new file mode 100644
index 0000000..f5400e8
--- /dev/null
+++ b/include/webdav_cache_validation.inc.php
@@ -0,0 +1,66 @@
+<?php
+if (!defined('PHPWG_ROOT_PATH'))
+{
+  die('Hacking attempt!');
+}
+
+function bratonien_tools_webdav_derivative_cache_valid($target_path, $derivative, $params=null, &$reason=null)
+{
+  $reason = '';
+  $target_path = (string)$target_path;
+  if ($target_path === '' || !is_file($target_path) || !is_readable($target_path))
+  {
+    $reason = 'missing';
+    return false;
+  }
+
+  $cache_root = PHPWG_ROOT_PATH.PWG_DERIVATIVE_DIR;
+  if (strpos($target_path, $cache_root) !== 0)
+  {
+    $reason = 'outside-cache';
+    return false;
+  }
+
+  clearstatcache(true, $target_path);
+  $bytes = @filesize($target_path);
+  if ($bytes === false || $bytes < 1)
+  {
+    $reason = 'empty';
+    return false;
+  }
+
+  $actual = @getimagesize($target_path);
+  if (!is_array($actual) || (int)($actual[0] ?? 0) < 1 || (int)($actual[1] ?? 0) < 1)
+  {
+    $reason = 'not-image';
+    return false;
+  }
+
+  if (is_object($derivative) && method_exists($derivative, 'get_size'))
+  {
+    $expected = $derivative->get_size();
+    $expected_width = (int)($expected[0] ?? 0);
+    $expected_height = (int)($expected[1] ?? 0);
+    if ($expected_width > 0 && $expected_height > 0)
+    {
+      if ((int)$actual[0] !== $expected_width || (int)$actual[1] !== $expected_height)
+      {
+        $reason = 'dimension-mismatch:'.(int)$actual[0].'x'.(int)$actual[1].'!='.$expected_width.'x'.$expected_height;
+        return false;
+      }
+    }
+  }
+
+  if (is_object($params) && isset($params->last_mod_time))
+  {
+    $mtime = @filemtime($target_path);
+    if ($mtime === false || (int)$mtime < (int)$params->last_mod_time)
+    {
+      $reason = 'stale-params';
+      return false;
+    }
+  }
+
+  $reason = 'valid';
+  return true;
+}