Point the Authorize.net integration to a different end point (e.g. to use an Authorize.net mirror API)

my_pmpro_authorizenet_post_url.php

function my_pmpro_authorizenet_post_url($url, $environment)
{
  if($environment == "sandbox")
    $url = "https://domain.com/path-to-the-sandbox-endpoint/";
  else
    $url = "https://domain.com/path-to-the-live-endpoint/";

  return $url;
}
add_filter('pmpro_authorizenet_post_url', 'my_pmpro_authorizenet_post_url', 10, 2);