If there is a get request that then displays the value after server-side processing via PHP, ex.
mysite.com?message=, is it possible to pass in a value which gets variables from the PHP form which processes it?
Depending on the PHP script on the other site this might be possible, for example if the message is processed by PHP to explicitly retrieve the content of variables based on the content of the message. But there is no general way which works against an arbitrary PHP script. And in any case this would not be called XSS since XSS is used to describe the injection of script on the client side (browser) which is not the case here.