Gravity forms - better inventory

<?php
/*
Plugin Name: My required plugins
Plugin URI: http://mysite.com/
Description: functions that I want active on all themes
Author: My name
Version: 1.0.0
Author URI: http://mysite.com
*/

/**
* Better Inventory with Gravity Forms / Limit by Sum of Field Values
* http://gravitywiz.com/2012/05/19/simple-ticket-inventory-with-gravity-forms/
*/

class GWLimitBySum {

    private $_args;

    function __construct($args) {

        $this->_args = wp_parse_args($args, array(
            'form_id' => false,
            'field_id' => false,
            'limit' => 20,
            'limit_message' => __('Sorry, this item is sold out.'),
            'validation_message' => __('You ordered %1$s of this item. There are only %2$s of this item left.'),
            'approved_payments_only' => false,
            'hide_form' => false
            ));

        $this->_args['input_id'] = $this->_args['field_id'];
        extract($this->_args);

        add_filter("gform_pre_render_$form_id", array(&$this, 'limit_by_field_values'));
        add_filter("gform_validation_$form_id", array(&$this, 'limit_by_field_values_validation'));

        if($approved_payments_only)
            add_filter('gwlimitbysum_query', array(&$this, 'limit_by_approved_only'));

    }

    public function limit_by_field_values($form) {

        $sum = self::get_field_values_sum($form['id'], $this->_args['input_id']);
        if($sum < $this->_args['limit'])
            return $form;

        if($this->_args['hide_form']) {
            add_filter('gform_get_form_filter', create_function('', 'return "' . $this->_args['limit_message'] . '";'));
        } else {
            add_filter('gform_field_input', array(&$this, 'hide_field'), 10, 2);
        }

        return $form;
    }

    public function limit_by_field_values_validation($validation_result) {

        extract($this->_args);

        $form = $validation_result['form'];
        $exceeded_limit = false;

        foreach($form['fields'] as &$field) {

            if($field['id'] != intval($input_id))
                continue;

            $requested_value = rgpost("input_" . str_replace('.', '_', $input_id));
            $field_sum = self::get_field_values_sum($form['id'], $input_id);

            if($field_sum + $requested_value <= $limit)
                continue;

            $exceeded_limit = true;
            $number_left = $limit - $field_sum >= 0 ? $limit - $field_sum : 0;

            $field['failed_validation'] = true;
            $field['validation_message'] = sprintf($validation_message, $requested_value, $number_left);

        }

        $validation_result['form'] = $form;
        $validation_result['is_valid'] = !$validation_result['is_valid'] ? false : !$exceeded_limit;

        return $validation_result;
    }

    public function hide_field($field_content, $field) {

        if($field['id'] == intval($this->_args['input_id']))
            return "<div class=\"ginput_container\">{$this->_args['limit_message']}</div>";

        return $field_content;
    }

    public static function get_field_values_sum($form_id, $input_id) {
        global $wpdb;

        $query = apply_filters('gwlimitbysum_query', array(
            'select' => 'SELECT sum(value)',
            'from' => "FROM {$wpdb->prefix}rg_lead_detail ld",
            'where' => $wpdb->prepare("WHERE ld.form_id = %d AND CAST(ld.field_number as unsigned) = %d", $form_id, $input_id)
            ));

        $sql = implode(' ', $query);
        return $wpdb->get_var($sql);
    }

    public static function limit_by_approved_only($query) {
        $query['from'] .= ' INNER JOIN wp_rg_lead l ON l.id = ld.lead_id';
        $query['where'] .= ' AND l.payment_status = \'Approved\'';
        return $query;
    }

}

new GWLimitBySum(array(
    'form_id' => 113,
    'field_id' => 13.3,
    'limit' => 8,
    'limit_message' => 'Sorry, there are no more tickets!',
    'validation_message' => 'You ordered %1$s tickets. There are only %2$s tickets left.',
    'approved_payments_only' => false,
    'hide_form' => false
    ));