Nilsan, скрипты не нужны
<form name="Dohod" method="get" action="getxml.php" target="getxml">
<!-- ..... -->
<input type="text" name="rezultdohod" value="">
<input type="submit" value="Скачать XML">
</form>
<iframe name="getxml" src="" style="display:none"></iframe>
getxml.php
<?php
header('Content-type: text/xml; charset=utf8');
header('Content-disposition: attachment; filename='.date('YmdHis').'.xml');
echo '<?xml version="1.0" encoding="utf-8"?><stroka1>'.$_GET['rezultdohod'].'</stroka1>';
?>