Create User at sendinblue.com via sendinblue/APIv3-php-library

create_user.php

<?php
function create_user($sendinblueApiKey = '', $mail = '', $sendinblueLists = '1,2,3'){
  $config = \SendinBlue\Client\Configuration::getDefaultConfiguration()->setApiKey('api-key', $sendinblueApiKey);

  $apiInstance = new \SendinBlue\Client\Api\ContactsApi(
      new \GuzzleHttp\Client(),
      $config
  );

  $listIds = [];
  foreach (explode(',', $sendinblueLists) as $listId){
      $listIds[] = (int)trim($listId);
  }

  $createContact = new \SendinBlue\Client\Model\CreateContact([
      'email' => $mail,
      'updateEnabled' => true,
      'attributes' =>
          [
              'VORNAME' => 'name', /* Attribute name in sendinblue */
              'NACHNAME' => 'name', /* Attribute name in sendinblue */
          ],
      'listIds' => $listIds

  ]);

  $result = $apiInstance->createContact($createContact);
  if(\is_a($result, \SendinBlue\Client\Model\CreateContact::class )){
      $sendinblue_id = $result->getId();
  } else {
      $user_info = $apiInstance->getContactInfo($lead->getEmail());
      $sendinblue_id = $user_info->getId();
  }
  
  return $sendinblue_id;
}