Adding PHP Into Javascript Output

I want to dispaly different things if the screen size is different.

I have this

<script language="javascript">
if(window.screen.width <= 800)
{
document.write("small");
}
else
{
document.write("large");
}
</script>

I want to add PHP within the document.write, how would I do that? For example I want to do this:

<script language="javascript">
if(window.screen.width <= 800)
{
document.write("small");
}
else
{
document.write("large <?php echo $variable; ?> more text");
}
</script>