What do you mean? If you mean one the user types in the message and clicks the prompt, and you want it in the page then... its aint gunna happen. JS is client side, PHP is server side, always keep this in mind. The only way to make JS $message go back to PHP is by sending the $message as a header or sending the variable back to the server, process it and the server would send it back. You could do something like this if you wanted I guess
document.location = "http://mydomain.com/nextpage.php?message=" . message;
My JS is a little hazy... but I think this would send something back to the server, in JS.