If you insist on two files (though I agree with Allen and Rod - put the form in message.php), you can do this:
message.php:
<?php
...process your data WITH NO OUTPUT TO BROWSER
header("Location: message.html");
?>
That's it. Nothing may be outside <?php ?> block!
And in message.html, use:
<form action="message.php">...
target="..." is used with frames.