Hi all:

This is the first time I am working for a front-end project that requires server-side authentication for AJAX requests. I've encountered problems like I cannot make a call of session_start as the beginning line of the "destination page", cuz that would get me a PHP Warning :

Warning: session_start() [function.session-start]: 
Cannot send session cache limiter -
headers already sent (output started at C:\xampp\htdocs\comic\app\ajaxInsert
    Book.php:1)
in C:\xampp\htdocs\comic\app\common.php  on line 10

I reckon this means I have to figure out a way other than checking PHP session variables to authenticate the "caller" of this PHP script, and this is my approach :

I have a "protected" PHP page, which must be used as the "container" of my javascript that posts the form through jQuery $.ajax(); method

In my "receiver" PHP script, what I've got is:

<?php
define(BOOKS_TABLE, "books");
define(APPROOT, "/comic/");
define(CORRECT_REFERER, "/protected/staff/addBook.php");

function isRefererCorrect()
{
    // the following line evaluates the relative path for the referer uri, 
    // Say, $_SERVER['HTTP_REFERER'] returns "http://localhost/comic/protected/staff/addBook.php"
    // Then the part we concern is just this "/protected/staff/addBook.php"
    $referer = substr($_SERVER['HTTP_REFERER'], 6 + strrpos($_SERVER['HTTP_REFERER'], APPROOT));
    return (strnatcmp(CORRECT_REFERER, $referer) == 0) ? true : false;
}

//http://stackoverflow.com/questions/267546/correct-http-header-for-json-file
header('Content-type: application/json charset=UTF-8');
header('Cache-Control: no-cache, must-revalidate');

echo json_encode(array
    (
        "feedback"=>"ok", 
        "info"=>isRefererCorrect()
    ));
?>

My code works, but I wonder is there any security risks in this approach? Can someone manipulate the post request so that he can pretend that the caller javascript is from the "protected" page?

Many thanks to any hints or suggestions.