PHP check http referer for form submitted by AJAX, secure?

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开发者_如何学JAVA/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?

UPDATE:

just realized I can let javascript from the secured page generate a unique token per ajax request, and use the passed token value to authenticate whether it is a "genuine ajax call" from the secured page

Will this be much better? Or should I just encrypt the content of the post request?

UPDATE AGAIN :

After two hours of looping through the included pages, I finally noticed that this weird situation was caused by my PHP page encoding...

I gave Notepad++ a try and carelessly chose the page encoding as UTF-8 with Byte Order Marker, so I kept getting the warning message due to the "weird" interpretation of this line:

<?php

A good lesson for me...

Many thanks to any hints or suggestions.

Technically seen, somebody can send you a request with referrer being 127.0.0.1, 0.0.0.0 or pretty much whatever they want without ever touching the site; so yeah, relying on it might not be the best approach.

Also: You CAN and SHOULD use php_session - you simply have to call session_start(); BEFORE anything else sends ANYTHING (as the error clearly states: headers already sent). Judging from the error

(output started at C:\xampp\htdocs\comic\app\ajaxInsert Book.php:1)

this might simply be a newline at the beginning of the file i.e

//empty line
<?php
//code

Ad. Update: It'll probably make things more secure; given the page which creates the token can only be accessed by people with the required permissions; it seems ok. Though even after reading your comment I believe you have some unneccessary whitespace somewhere, as sessions seem a perfectly fine way to authenticate the AJAX request.