Update table after onclick php - php

Question

i build system notifications and have me problem.
i want update table 'notifications' after onclick from 'new=1' to new='0' for a user logged.
how i update at database after onclick with ajax or something other.
if i send request server with ajax as this:
function readNotifications(str)
{
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","controlpanel.php?notification=readNews",true);
xmlhttp.send();
}
The server does not know which USER ID to update the alerts.
And if I send USER ID at time be possible to change the USER ID and everyone can update the notifications of each other and this a not security.
I'm sorry for my English, if I do not understand you can ask a question.
Thank you!

Answer 1

I think its better for you to save the user_id in a session variable once the user logs in.
<?php
session_start();
// store session data
$_SESSION['user_id']=100001;
?>
and access it from your code behind during the processing of your ajax request.
$_SESSION['user_id']
AND
if you use cookies, you can still accept that from your server side code.
$_COOKIE["user_id"];
will do it.
just make sure that, you set the withCredentials property to true.
var req= new XMLHttpRequest();
req.withCredentials = true;