Add docs for casting functions

en/appendices/5-1-upgrade-guide.rst

@@ -32,6 +32,13 @@ Console
 
 - ``Arguments::getBooleanOption()`` and ``Arguments::getMultipleOption()`` were added.
 
+Core
+----
+
+- The ``toString``, ``toInt``, ``toBool`` were added. These methods give you
+  a typesafe way to cast request data or other input. Should casting not
+  succeed, ``null`` will be returned.
+
 Http
 ----
 

en/controllers/request-response.rst

@@ -100,6 +100,25 @@ If you want to access all the query parameters you can use
 
     $query = $this->request->getQueryParams();
 
+You can use the casting utility functions to provide typesafe access to request
+data and other input::
+
+    use function Cake\Core\toBool;
+    use function Cake\Core\toInt;
+    use function Cake\Core\toString;
+
+    // $active is bool|null.
+    $active = toBool($this->request->getQuery('active'));
+
+    // $page is int|null.
+    $page = toInt($this->request->getQuery('page'));
+
+    // $query is string|null.
+    $query = toString($this->request->getQuery('query'));
+
+.. versionadded:: 5.1.0
+    Casting functions were added.
+
 Request Body Data
 -----------------