how to use javascript variable in php

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

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

<script>
var p1 = "success";
</script>

<?php
echo "<script>document.writeln(p1);</script>";
?>

<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>

<?php
$bool = false;
$num = 3 + 4;
$str = "A string here";
?>

<?php
    $color = "Red";
?>
<script type="text/javascript">var color = "<?= $color ?>";</script>
<script type="text/javascript" src="file.js"></script>

alert("color: " + color);