Documentations of Library

Form Validation Documentation CodeIgniter 3.x <http://codeigniter.com/userguide3/libraries/form_validation.html>_
:doc:Validation Documentation CodeIgniter 4.x </libraries/validation>

What has been changed

If you want to change validation error display, you have to set CI4 :ref:validation View templates <validation-customizing-error-display>.
CI4 validation has no Callbacks <http://www.codeigniter.com/userguide3/libraries/form_validation.html#callbacks-your-own-validation-methods>_ in CI3. Use :ref:Callable Rules <validation-using-callable-rule> (since v4.5.0) or :ref:Closure Rules <validation-using-closure-rule> (since v4.3.0) or :ref:Rule Classes <validation-using-rule-classes> instead.
In CI3, Callbacks/Callable rules were prioritized, but in CI4, Closure/Callable Rules are not prioritized, and are checked in the order in which they are listed.
Since v4.5.0, :ref:Callable Rules <validation-using-callable-rule> has been introduced, but it is a bit different from CI3's Callable <http://www.codeigniter.com/userguide3/libraries/form_validation.html#callable-use-anything-as-a-rule>_.
CI4 validation format rules do not permit empty string.
CI4 validation never changes your data.
Since v4.3.0, :php:func:validation_errors() has been introduced, but the API is different from CI3's.

Upgrade Guide

Within the view which contains the form you have to change:
- <?php echo validation_errors(); ?> to <?= validation_list_errors() ?>
Within the controller you have to change the following:
- $this->load->helper(array('form', 'url')); to helper('form');
- remove the line $this->load->library('form_validation');
- if ($this->form_validation->run() == FALSE) to if (! $this->validateData($data, $rules)) where $data is the data to validate, typically, the POST data $this->request->getPost().
- $this->load->view('myform'); to return view('myform', ['validation' => $this->validator,]);
You have to change the validation rules. The new syntax is to set the rules as array in the controller:

.. literalinclude:: upgrade_validations/001.php

Code Example

CodeIgniter Version 3.x

Path: application/views::

<html>
<head>
    <title>My Form</title>
</head>
<body>

    <?php echo validation_errors(); ?>

    <?php echo form_open('form'); ?>

    <h5>Username</h5>
    <input type="text" name="username" value="" size="50" />

    <h5>Password</h5>
    <input type="text" name="password" value="" size="50" />

    <h5>Password Confirm</h5>
    <input type="text" name="passconf" value="" size="50" />

    <h5>Email Address</h5>
    <input type="text" name="email" value="" size="50" />

    <div><input type="submit" value="Submit" /></div>

    </form>

</body>
</html>

Path: application/controllers:

.. literalinclude:: upgrade_validations/ci3sample/002.php

CodeIgniter Version 4.x

Path: app/Views::

<html>
<head>
    <title>My Form</title>
</head>
<body>

    <?= validation_list_errors() ?>

    <?= form_open('form') ?>

    <h5>Username</h5>
    <input type="text" name="username" value="" size="50" />

    <h5>Password</h5>
    <input type="text" name="password" value="" size="50" />

    <h5>Password Confirm</h5>
    <input type="text" name="passconf" value="" size="50" />

    <h5>Email Address</h5>
    <input type="text" name="email" value="" size="50" />

    <div><input type="submit" value="Submit" /></div>

    </form>

</body>
</html>

Path: app/Controllers:

.. literalinclude:: upgrade_validations/002.php