<?php
    $response = $api->updateStock(
      array(
        array(
          'id' => 414021,
          'fields' => array(
            array(
              'id' => 121,
              'value' => array(
                array(
                  // insert, delete
                  'mode'        => 'insert',
                  'attach_type' => 'customer',
                  'attach_id'   => 556066
                )
              )
            )
          )
        )
      )
    );