php中的ajax新记录计数通知

iam doing a page in php that if any new record is entered it will notify the users screen with the new record count. Following is the code i did for the same, but its not working fine. Can u pls suggest me as of what iam doing wrong...

 alert.php

 <?php
 require("config.php");
 $result = mysql_query("SELECT * FROM marketing_tend_corr");
 $res = mysql_num_rows($result);
 echo $res;
 ?>




 index.php
 <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
 <?php 
 define('BASEPATH', true);
 require("config.php");
 ?>
 <script>

 var count_cases = -1;
 setInterval(function(){    
    $.ajax({
    type : "POST",
    url : "alert.php",
    success : function(response){
        if (count_cases != -1 && count_cases != response) echo $count_cases);
        count_cases = response;
    }
});
 },1000);

 </script>

The following line of code is not going to work in Javascript:

if (count_cases != -1 && count_cases != response) echo $count_cases);

This line of code contains php code (echo $count_cases) which is server side code.

I've changed the code a bit and replaced the number of records by returning a random value.

// alert.php

<?php

echo rand(1, 1000000);

//index.php

<!DOCTYPE html>
<html>
<head>
    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
    <script type="text/javascript">

        setInterval(function(){
            $.ajax({
                type : "POST",
                url : "alert.php",
                success : function(response){                        
                    $("body").html(response);
                }
            });
        },1000);

    </script>
</head>
<body>
</body>
</html>

You can check the index.php file in your browser to see the random numbers being returned. This random number should in your case become the result of your 'mysql_num_rows' function.