The interface for the HC On-line payment method is a REST API, where JSON notation is used for all communication (queries - requests and responses) and data description.
The Technical Documentation for the API (hereinafter TD) and basic description of the payment method functionality can be found at https://csoneclicknew.docs.apiary.io/#
This guide to the public API including tutorials and sample data
Installment calculator in the form of a JavaScript widget (widget-calculator)
PHP script providing basic functionality necessary for integrating HC payment into your e-shop (php-script)
PHP library providing basic functionality necessary for integrating HC payment into your e-shop (oneclick-api-client) - WARNING - DEPRECATED - only for PHP 5.6!!!