Very sorry that I saw your message now.
Actually I solved this issue by some other way, what I did is:
When I need to send data to PHP website, I save the data into a buffer table(publish as web service) and use function "HYPERLINK" to call PHP URL, when PHP got that URL(will key value in url), PHP analyse it and get data from buffer table via web service, after PHP processed, PHP will raise NAV client's URL and open particular card or list page in NAV.
When PHP need to send data to NAV, it will first save data to NAV's buffer table via web service, and then call function in NAV to process it.
When error happens in NAV, the error message will be captured by PHP and show in PHP page.