how to get the php variable in javascript code example

Example 1: javascript php variable

<script type="text/javascript">
   var php_var = "<?php echo $php_var; ?>";
</script>

Example 2: using js variable in php

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