JsonInputFilter.php

<?php

namespace App\Routing\Filter;

use Cake\Event\Event;
use Cake\Routing\DispatcherFilter;

class JsonInputFilter extends DispatcherFilter
{
    public function beforeDispatch(Event $e)
    {
        $request = $e->data['request'];
        if (!$request->is(['put', 'post', 'patch'])) {
            return;
        }

        if (strpos($request->header('Content-Type'), 'application/json') === false) {
            return;
        }

        $request->data = json_decode($request->input(), true);
    }
}