How to pass a variable / data from javascript to php and vice versa?

<!-- headers etc. omitted -->
<script>
function callPHP(params) {
    var httpc = new XMLHttpRequest(); // simplified for clarity
    var url = "get_data.php";
    httpc.open("POST", url, true); // sending as POST

    httpc.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    httpc.setRequestHeader("Content-Length", params.length); // POST request MUST have a Content-Length header (as per HTTP/1.1)

    httpc.onreadystatechange = function() { //Call a function when the state changes.
    if(httpc.readyState == 4 && httpc.status == 200) { // complete and no errors
        alert(httpc.responseText); // some processing here, or whatever you want to do with the response
        }
    }
    httpc.send(params);
}
</script>
<a href="#" onclick="callPHP('lorem=ipsum&foo=bar')">call PHP script</a>
<!-- rest of document omitted -->

<script language="javascript" type="text/javascript">
 function _vals(target, value){
   form1.all("target").value=target;
   form1.all("value").value=value;
   form1.submit();
 }
</script>

<a onclick="_vals('name', 'val')" href="#">My Link</a>

$val = $_POST['value'];

<script>
   var php_value = '<?php echo $php_variable; ?>';

   var infobar_active = $.cookie('php_value');
   var infobar_alert  = any_process(infobar_active);

    //set a cookie to readit via php
   $.cookie('infobar_alerta', infobar_alerta );

   </script>

   <?php

    var js_value = code to read a cookie

   ?>