Proper way to get array from session

I will show you an example of how you can get out of this situation.

PHP Code:

// route 
$routes->add('/', 'Home::index');
// in controller 
public function index()
{
    if ($this->request->getMethod() !== 'get') {
        if (! $this->validate(['a.c' => 'required'])) {
            return redirect()->to('/')->withInput();
        } else {
            d('win');
        }

    } else {
        session(); 
        return view('template', ['validator' => Services::validation()]);
    }
}
// template
<?php /** @var Codeigniter\Validation\Validation $validator */ ?>
<h1>error: <?= $validator->getError('a.c')?></h1>
<form method="post" action="/">
    <input type="text" name="a[b]" value="aa">
    <input type="submit">
</form> 


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Anyway, I added a PR to access the key containing the dot using dot notation.
https://github.com/codeigniter4/CodeIgniter4/pull/4588