获取用于AJAX的值变量PHP

Question

How can ajax can receive the value that was set in php

<?php
$a ="2";
?>

why the result awlays NaN ? I do not use the method post, get

<input type="text" id="as" value="<?php echo $a; ?>">
<script>
$(document).ready(function(){
    $("#act<?php echo $i; ?>").click(function(){
        var y = $("as").value;
        var w = y-1;

        $("#tr<?php echo $i;?>").empty();
        $("#as").val(w);
    });
});
</script>

Answer 1

var y = $("as").value; should be var y = $("#as").val();

Answer 2

$.ajax({
         type: "POST",
         url: "test.php",
         data: {
             "as": as
         },
         success: function(data) {
            alert(data);
         }
     });