Force the browser to cache a .php - php

Question

I need the browser to cache a large, mostly static .php file. I open it via ajax and want to add it to the current page.
After some research if found this
$seconds_to_cache = 3600;
$ts = gmdate("D, d M Y H:i:s", time() + $seconds_to_cache) . " GMT";
header("Expires: $ts");
header("Pragma: cache");
header("Cache-Control: max-age=$seconds_to_cache");
This works for IE, but not for chrome and firefox.
Here is the request
Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding gzip, deflate
Accept-Language de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Cache-Control max-age=0
Connection keep-alive
Content-Type application/x-www-form-urlencoded
Cookie PHPSESSID=5dkvr42f4it8pnnnqpesj6l413
Host localhost
Referer http://localhost/mifa/Suche.php
User-Agent Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20100101 Firefox/13.0.1
charset utf-8
and here the response header
Cache-Control max-age=3600
Connection Keep-Alive
Content-Type text/html
Date Thu, 05 Jul 2012 15:28:22 GMT
Expires Thu, 05 Jul 2012 16:28:22 GMT
Keep-Alive timeout=5, max=91
Pragma cache
Server Apache/2.2.21 (Win32) mod_ssl/2.2.21 OpenSSL/1.0.0e PHP/5.3.8 mod_perl/2.0.4 Perl/v5.10.1
Transfer-Encoding chunked
X-Powered-By PHP/5.3.8
What do i need to change?
EDIT
Apparently, only IE does not append the Cache-Control max-age=0 to the request.
Here is the JS Function of the request
url = "includes/Orte.php";
obj.onreadystatechange = rState;
obj.open("GET", url, true);
obj.setRequestHeader("Pragma", "");
obj.setRequestHeader("Cache-Control", "");
obj.setRequestHeader("charset", "utf-8");
obj.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
obj.setRequestHeader("Connection", "close");
obj.send();
function rState(){
if(obj.readyState == 4){
if (obj.status == 200){
//alert("Response Text Ajax:\n" + obj.responseText + "\nEnd Response Text");
}
}
}

Answer 1

The Cache-Control: max-age=0 header in the request means that you asked your browser to refresh the page, so he just ignores the cache.
Access the page without hitting refresh (e.g. focus the address bar and hit enter) to avoid this.
Also, if the page is on an HTTPS URL, you may have to add public to the Cache-Control header, else some browsers won't cache it.

Answer 2

Two things that come to mind are the last modified header, and using .htaccess cache control. The latter is for broad types but you can use it just for one folder, with that file in a folder by itself.
header("Last-Modified: ... ");