Cache Headers on PHP file not working - php

Question

I have an application where a number of otherwise static javascript files are being generated via PHP to allow configuration options to alter the static files (path like: mystaticfile.js.php). Everything works fine EXCEPT that I can't seem to get the cache settings to work and these resources are being reloaded with every page load.
The PHP file uses the following headers to try to set the cache settings:
$expires= 60 * 60 * 24 * 60; //cache for 60 days
header('Pragma: public');
header('Cache-Control: max-age=' . $expires);
header('Expires: ' . gmdate('D, d M Y H:i:s', time() + $expires) . ' GMT');
header("content-type: application/x-javascript");
However, when the files are served they're showing headers that look like:
HTTP/1.1 200 OK
Date: Sun, 06 Nov 2016 19:18:00 GMT
Server: Apache/2.2.15 (CentOS)
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 744
Keep-Alive: timeout=15, max=95
Connection: Keep-Alive
Content-Type: application/x-javascript
My first thought was that it was that it might be because Apache has the ExpiresActive flag set on but I don't see any ExpiresByType rules set for PHP files.
Reading online it sounds like ETag issues could be the problem, but I've added
Header unset Pragma
FileETag None
Header unset ETag
to the http.conf file (and restarted the service) and still no dice.
Any thoughts?

Answer 1

Source: PHP: Worry about some magical added “Cache-Control” Header ?
These headers are automatically set by the PHP Session module to
prevent browser/proxy based caching of your pages. Depending on your
environment setup, it’s possible to control these headers by using the
session_cache_limiter() function or use the php.ini
To disable these behaviour just pass an empty string to the
session_cache_limiter() function as mentioned in the documentation:
session_cache_limiter('');