You’ll find many of these proxies on Arduino Playground, including useful descriptions. They are written in languages better suited for the task (well some of them are) and act as intermediaries between serial and network connections. In PHP, you’d simply connect to the socket provided by the proxy (careful, some are unidirectional). This option might be your best bet on Windows, but obviously makes things more complicated.
This list is not exhaustive, but at this point I decided to abandon the entire idea. Don’t hesitate to post your solution in the comments if you’ve found one that isn’t listed. However, please don’t bother asking me any PHP-and-RS-232 related questions as I won’t be able to answer them.