Explain javascript and php code

I got this code from internet, it works correctly

<script type="text/javascript">
var jvalue = 'this is javascript value';
<?php $abc = "<script>document.write(jvalue)</script>"?>   
</script>
<?php echo  $abc;?>

But when I put the </script> at the end it stops working, kindly explain why it stops working?

<script type="text/javascript">
var jvalue = 'this is javascript value';
<?php $abc = "<script>document.write(jvalue)</script>"?>   
<?php echo  $abc;?>
</script>