think\Model::writeTransform PHP Method

    protected function writeTransform($value, $type)
    {
        if (is_array($type)) {
            list($type, $param) = $type;
        } elseif (strpos($type, ':')) {
            list($type, $param) = explode(':', $type, 2);
        }
        switch ($type) {
            case 'integer':
                $value = (int) $value;
                break;
            case 'float':
                if (empty($param)) {
                    $value = (double) $value;
                } else {
                    $value = (double) number_format($value, $param);
                }
                break;
            case 'boolean':
                $value = (bool) $value;
                break;
            case 'timestamp':
                if (!is_numeric($value)) {
                    $value = strtotime($value);
                }
                break;
            case 'datetime':
                $format = !empty($param) ? $param : $this->dateFormat;
                $value = date($format, is_numeric($value) ? $value : strtotime($value));
                break;
            case 'object':
                if (is_object($value)) {
                    $value = json_encode($value, JSON_FORCE_OBJECT);
                }
                break;
            case 'array':
                $value = (array) $value;
            case 'json':
                $option = !empty($param) ? (int) $param : JSON_UNESCAPED_UNICODE;
                $value = json_encode($value, $option);
                break;
            case 'serialize':
                $value = serialize($value);
                break;
        }
        return $value;
    }