how to set javascript variables via php code example

Example 1: 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;?>

Example 2: how to use php variable in javascript file

<script type="text/javascript">
// boolean outputs "" if false, "1" if true
var bool = "<?php echo $bool ?>"; 

// numeric value, both with and without quotes
var num = <?php echo $num ?>; // 7
var str_num = "<?php echo $num ?>"; // "7" (a string)

var str = "<?php echo $str ?>"; // "A string here"
</script>