[Drupal] How to create a payment gateway module for Drupal?

Drupal commerce is a popular e-commerce module for Drupal. It is used to build ecommerce websites and applications and leverages the outstanding features of Drupal 7 for greater flexibility. Here, we will find out how to create a payment gateway module for Drupal commerce. There are mainly two types of payment gateway integration in Drupal Commerce:

1. On site
2. Off site

On site payment is processed by a third party API through web services with the information gathered from our site through a checkout form (For example Credit/Debit card credentials).

Offsite payment method, on the other hand, processes data entered in the payment gateway page. Off site payment method redirects the user to the payment gateway's processing page using a checkout form. After the payment is completed, user will be redirected to payment page from where he can go forward or backward during the checkout process.

Payment gateways offer multiple types of payment services, which often provides on-site and off-site options. Each of these services is represented by a different payment method in Drupal Commerce.

A default payment method rule will be defined, when a payment method module is enabled. This could be used to configure your payment methods. During the checkout process, all the active payment method rules will be evaluated and the respective payment methods would be enabled for use on the checkout form. You can actually use one or more rules to enable any given payment method using a different set of API credentials or transaction configurations based on conditions of the order being paid for. Note that, no payment method is enabled by more than one rule.

Commerce payment module in the Drupal commerce provides API for creation of payment methods in Drupal commerce. The Payment module includes functions designed to support common types of payment services, the most common being credit card payments. The file commerce_payment.credit_card.inc includes helper functions for building and validating credit card forms and data.

Whenever a payment is done, a payment transaction should be created that references to the order for which payments were attempted and the payment status should be reflected in the transaction's status.

For reference of creating a payment gateway module, we can refer commerce_paypal as it contains both on-site and off-site payment methods.

Now, a payment method should contain the following functions.

1. Define the payment method via hook_commerce_payment_method_info(). This hook will return a keyed array like the one below:
   

   $payment_methods['paypal_wps'] = array(
    'base' => 'commerce_paypal_wps',
    'title' => t('PayPal WPS'),
    'short_title' => t('PayPal'),
    'description' => t('PayPal Website Payments Standard'),
    'terminal' => FALSE,
    'offsite' => TRUE,
     'offsite_autoredirect' => TRUE,
   );

   Each element in the array is explained in the link payment-info-hooks.

2. Define your callback functions to add setting form to the payment method's rule. Collect the necessary information on the checkout and administrative payment forms, and accommodate the redirection process for off-site payment methods.
3. Define menu items for additional payment transaction operations and providing the forms and API integration necessary to perform the operations.
4. Integrate with the payment service to actually process payments, validate payment notifications, and otherwise interact with the available APIs.

On Site Payments

A simple example of on-site payment method is the Paypal WPP module in the Commerce Paypal project. Here it is important to note that you have to set off-site parameter in the payment method data to FALSE and off-site_redirect to FALSE.

The function callbacks are defined in the callbacks key in the payment method array. For on-site payments, submit_form, submit_form_validate, and submit_form_submit functions need to be defined as it shows returns form elements to collect details from the customer required to process the payment and validates.

Off Site Payments

A simple example of on-site payment method is the Paypal WPS module in the Commerce Paypal project.

You have to set off-site parameter in the payment method data to TRUE to enable redirection to payment gateway. off-site_redirect to TRUE if you are redirecting to payment gateway page, FALSE if you are using iframe to show the payment method page.

The function callbacks are defined in the callbacks key in the payment method array. For off-site payments, redirect_form, redirect_form_validate, and redirect_form_submit functions are needed.

redirect_form - This function returns form elements which should be submitted to the redirected payment service. It is because of the array merge that happens upon return and the service’s URL that should receive POST variables. The POST variables should be set in the #action property of the returned form array.

redirect_form_validate - This function returns from a redirected payment service. This callback provides an opportunity for the payment method to validate any returned data before proceeding to checkout completion; should return TRUE or FALSE indicating whether or not the customer should proceed to checkout completion or go back a step in the checkout process from the payment page

redirect_form_submit - This function returns from a redirected payment service. This callback gives an opportunity for the payment method to perform any submission functions necessary before the customer is redirected to checkout completion.

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