Note that the Order api has been an adequate replacement for over a year - per efforts by wordpress integration
/**
* Process a transaction and record it against the contact.
*
+ * @deprecated
+ *
* @param array $params
* Input parameters.
*
* contribution of created or updated record (or a civicrm error)
*/
function civicrm_api3_contribution_transact($params) {
+ CRM_Core_Error::deprecatedFunctionWarning('The contibution.transact api is unsupported & known to have issues. Please see the section at the bottom of https://docs.civicrm.org/dev/en/latest/financial/OrderAPI/ for getting off it');
// Set some params specific to payment processing
// @todo - fix this function - none of the results checked by civicrm_error would ever be an array with
// 'is_error' set