Allow access to PHP file only through ajax on local server

Question

I have a website that needs to increment values in a database based upon user interaction. When users click a button a php script is called that increments the value. I'd like to protect this script from being accessed by outside scripts. Currently a user could write their own web page with a javascript function that hits the same php file repeatedly to blow up the value in the database.

Here's my jquery code that does the incrementing:

jQuery(function(){
$('.votebtn').click(function(e){
    var mynum = $(this).attr('id').substring(0,5);
    $.ajax({
            url:"countvote.php",
            type:"GET",
            data: { 
                thenum:mynum
            },
            cache: false,
            success:function(data) {
                alert('Success!');
                }
            }
        });
});
});

How would I go about making it so that only a call from ajax/jquery on the local server can access 'countvote.php'? If that's not the correct way to go about it, I'm open to any suggestion that will prevent my php script from being abused by outside scripts.

Answer 1

The solution needs two steps.

Firstly the ajax file must allow access only in ajax request with this code.

define('IS_AJAX', isset($_SERVER['HTTP_X_REQUESTED_WITH']) &&      strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest');
if(!IS_AJAX) {die('Restricted access');}

Secondly the ajax file has access in the name of file that call it with command $_SERVER['HTTP_REFERER']. So you can restrict access only in the host server.

$pos = strpos($_SERVER['HTTP_REFERER'],getenv('HTTP_HOST'));
if($pos===false)
  die('Restricted access');

Maybe the code can work only with the second part

Answer 2

You can check if $_SERVER['HTTP_X_REQUESTED_WITH'] equals xmlhttprequest, but it's not a reliable method to determine whether a request is an AJAX request or not, there is always a way to get around this. But it protects you from random hits like wrongly entered urls, crawlers etc.