<?php

if (!empty($_POST['param'])){
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename=' .$_POST['filename'] );
    echo "Poslal jsi: ".$_POST['param'];
    exit;
}

?>
<form method="POST">
    <input type="text" name="param" value="neco" />
    <input type="text" name="filename" value="soubor.txt" />
    <input type="submit" />
</form>