PHP在AJAX内调用

A PHP page contains a variable $hash and has a button. The button's click event is handled by jQuery. I want jQuery to do an AJAX call as follows:

$.post("testupdate.php",  {hash:"<?php echo $hash; ?>"} , function(data) {

           $("#body").html(data);
});   

However, the PHP code <?php echo $hash; ?> inserted inside the AJAX call isn't working. How can I make it work please? Thanks.

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Update: I made it work by inserting a script element in the html page which is processed before the jQuery call:

<script> var hash = "<?php echo $hash; ?>"; </script>

And then did the following in the jQuery function call:

$.post("testupdate.php",  {hashpost:hash} , function(data) {

           $("#body").html(data);
});