A simple class fo CodeIgniter that helps to receive JSON format data

Input_Json.php

<?php
/**
 * Author: Antonio Di Pinto
 * Email: adp19911229@gmail.com
 *
 * @description: A simple class fo CodeIgniter that helps to receive JSON format data
 */

class Input_Json extends CI_Input
{
    /**
     * @var array
     */
    protected $_validation_fields = [];

    /**
     * @var array
     */
    protected $_errors = [];

    /**
     * @var string
     */
    protected $_error_message = "Missing value: %s";

    /**
     * Input_Json constructor.
     */
    public function __construct()
    {
        parent::__construct();
    }

    /**
     * @param $field string Json key to retrieve
     */
    public function add_required_field($field)
    {
        $this->_validation_fields[] = $field;
    }

    public function set_error_message($message){
        $this->_error_message = $message;
    }

    /**
     * @return bool
     */
    public function has_required_fields()
    {

        foreach ($this->_validation_fields as $index) {
            if (is_null_or_empty($this->key($index))) {
                $this->_add_validation_error($index);
            }
        }

        return empty($this->_errors);
    }

    /**
     * @param null $index
     * @return mixed|null
     */
    public function key($index = null)
    {
        $input = parent::__get('raw_input_stream');

        if (!is_null_or_empty($input) && is_json($input)) {
            $json = json_decode($this->security->xss_clean($input), true);

            if (!empty($json)) {
                if (!is_null($index)) {
                    return isset($json[$index]) ? $json[$index] : null;
                }
                return $json;
            }
        }
        return null;
    }

    /**
     * @param $field
     */
    private function _add_validation_error($field)
    {
        $this->_errors[] = sprintf($this->_error_message, $field);
    }

    /**
     * @return array
     */
    public function errors()
    {
        return $this->_errors;
    }

}