如何从codeigniter中的json数组中读取

Im trying to decode json and get value from the json code.

[{
    "restaurant_id":1,
    "menu_template_id":2,
    "add_food_item_a":1,
    "menu_category_id":1,
    "status":0
} ,
{
    "restaurant_id":1,
    "menu_template_id":2,
    "add_food_item_a":2,
    "menu_category_id":1,
    "status":0
}]

i need to read from this json and create an array with add_food_item_a and status.

currently I'm using like this

public function readJson()
    {
        $json_obj = json_decode('');
        if (isset($GLOBALS["HTTP_RAW_POST_DATA"]) && !empty($GLOBALS["HTTP_RAW_POST_DATA"])) {
            $json_text = $this->cleanMe($GLOBALS["HTTP_RAW_POST_DATA"]);
            // now insert into user table
            $json_obj = json_decode($json_text);
        }
        return $json_obj;
    }

and I call this function like

$add_food_item_a = isset($json_obj->add_food_item_a) ? $json_obj->add_food_item_a : '';

but can't read from this array of json code