计算php中的每个onclick事件

Question

JS

<script> 
    function myAjax() {
      $.ajax({
           type: "POST",
           url: 'ajax.php',
           data:{action:'call_this'}
      });
</script>

HTML

<a href="'. $row['link'] .'"target="_blank" onclick="myAjax()">

PHP

<?php
    require ("static/php/inc/betaconfig.inc.php");
    if($_POST['action'] == 'call_this') {
        $conn->query("UPDATE users SET clicks = (clicks + 1) WHERE id = 13");   
    }
?>

Normally it should be $uid instead of 13 but I don't know how to do that too... First of all, why this doesnt work ? 2nd $uid is $id = $_REQUEST['id']; and then

$sql = "SELECT * FROM `link` WHERE id = '$id'";
$result4 = $conn->query($sql);

while($row = $result4->fetch_assoc()) {
    $uid = $row['userid'];
}

so the best thing that instead of 13 I could use $uid