docs: add "Redirect and Validation Errors"

Author: kenjis

user_guide_src/source/helpers/form_helper.rst

@@ -534,7 +534,7 @@ The following functions are available:
     that are stored in the session. To store the errors in the session, you need to use ``withInput()`` with :php:func:`redirect() <redirect>`.
 
     The returned array is the same as ``Validation::getErrors()``.
-    See :ref:`Validation <validation-getting-all-errors>` for details.
+    See :ref:`Validation <validation-redirect-and-validation-errors>` for details.
 
     Example::
 

user_guide_src/source/libraries/validation.rst

@@ -538,6 +538,25 @@ When specifying a field with a wildcard, all errors matching the mask will be ch
 
 .. literalinclude:: validation/029.php
 
+.. _validation-redirect-and-validation-errors:
+
+Redirect and Validation Errors
+==============================
+
+PHP shares nothing between requests. So when you redirect if a validation fails,
+there will be no validation errors in the redirected request because the validation
+has run in the previous request.
+
+In that case, you need to use Form helper function :php:func:`validation_errors()`,
+:php:func:`validation_list_errors()` and :php:func:`validation_show_error()`.
+These functions check the validation errors that are stored in the session.
+
+To store the validation errors in the session, you need to use ``withInput()``
+with :php:func:`redirect() <redirect>`:
+
+.. literalinclude:: validation/042.php
+   :lines: 2-
+
 .. _validation-customizing-error-display:
 
 *************************

user_guide_src/source/libraries/validation/042.php

@@ -0,0 +1,6 @@
+<?php
+
+// In Controller.
+if (! $this->validateData($data, $rules)) {
+    return redirect()->back()->withInput();
+}