PHP未定义的查询参数

Question

Im using ajax to pass params through a post. Pretty much my issue is that when i recieve the params in PHP if i do a isset($_POST['param']) it returns true and if i echo the param it displays undefined. The param im passing through ajax is not set which is correct in this case. The idea would be when i check if the param is set for it to return false if it is undefined, however this isnt happening. Any specific reasons as to why this might be?

$param= $_POST['param'];

$concat = array();
if(!empty($param) && isset($param)){ $concat[] = "param= '" . mysql_real_escape_string($param) . "'"; }

The above is my actual code ive done echos on the original $_POST['param']; and also on $param to see if this was the problem but it results the same. the echos ive done are of the actual param/variable and of the result of isset on these two.

Thanks in advance!

UPDATE

ajax

$.ajax({
        type: "POST",
        url: "ajax/update.php",
        dataType: "text",
        data: "param=" + param,
        async: false,
        success: function(response) {
            if(response)
            {
                alert(response);
            }
        }
    });

param is defined above but in this specific case is not being set to anything it remains empty, passing an empty var.