[keyness]

Oh, that's right, I'll try it. Thank you Chris!

By the way, after asked this question I managed to do it with iframe like this:

<script type="text/javascript">
if (screen.width<=1280)
{
 document.write('<iframe src="headersm.php" scrolling="no" frameborder="0" style="border:none; overflow:hidden; width:100%; height:260px;"></iframe>');
}
else
{
 document.write('<iframe src="headerbig.php" scrolling="no" frameborder="0" style="border:none; overflow:hidden; width:100%; height:260px;"></iframe>');
}
</script>

However, without iframe it would be better of course.