PHP Dynamic Files Caching Problem

I have some CSS and JS files that are dynamically generated by PHP using this in my .htaccess:

AddHandler application/x-httpd-php .css .js

And then inside the files, for example:

<?PHP if ($Browser == 'msie') { ?>
.bind('selectstart', function(event) { [...] })
<?PHP } ?>

On the top of them (first line ) i use a conditional get based on Last-Modified header, like this:

<?PHP if (FileIsCached(getlastmod())) die(); ?>

This is the function:

public static function FileIsCached($Timestamp)
{
    $LastModified = substr(date('r', $Timestamp), 0, -5).'GMT';
    $IfModifiedSince = (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : false);

    if (($Semicolon = strrpos($IfModifiedSince, ';')) !== false)
        $IfModifiedSince = substr($IfModifiedSince, 0, $Semicolon);

    header('Last-Modified: '.$LastModified);

    if (!$IfModifiedSince || ($IfModifiedSince != $LastModified))
        return false;

    header('HTTP/1.1 304 Not Modified');

    return true;
}

I don't need to check encoding because I use automatic gzipping inside my php.ini:

 output_handler = ob_gzhandler

I put them normally inside my index.php like this:

<script type="<?PHP echo $JavascriptMIME; ?>" src="/script.js"></script>

Everything works like a charm... this is my first load response header:

HTTP/1.1 200 OK
Date: Fri, 16 Sep 2011 21:21:51 GMT
Last-Modified: Fri, 16 Sep 2011 19:25:37 GMT
Content-Encoding: gzip
Vary: Accept-Encoding
Cache-Control: max-age=31536000, public
Expires: Sat, 15 Sep 2012 21:21:51 GMT
Content-Type: application/javascript
Content-Length: 6161
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive

This is my cached response header:

HTTP/1.1 304 Not Modified
Date: Fri, 16 Sep 2011 21:24:39 GMT
Connection: Keep-Alive
Keep-Alive: timeout=5, max=100
Expires: Sat, 15 Sep 2012 21:24:39 GMT
Cache-Control: max-age=31536000, public
Vary: Accept-Encoding

The problem is that sometimes HTTP data seems to get messed up, maybe by gzipped content or something similar. Chrome network console sometimes show the following error: "Resource interpreted as Other but transferred with MIME type undefined". Mozilla sometimes makes 2 requests for index.php or try to download it with download prompt. Sometimes file requests stop at 5 when I normally load 7 files and the last downloaded file content is messed up like:

���������ÿÿ���������HTTP/1.1 200 OK
Date: Fri, 16 Sep 2011 19:53:26 GMT
Server: Apache/2.2.17 (Win32) PHP/5.3.6
X-Powered-By: PHP/5.3.6
Cache-Control: must-revalidate, no-cache, no-store, private, public
Cache-Control: max-age=0, max-stale=0, post-check=0, pre-check=0
Expires: Wed, 11 Apr 1984 18:36:00 GMT
Expires: 0
Last-Modified: Fri, 16 Sep 2011 19:53:26 GMT
Pragma: no-cache
Content-Encoding: gzip
Vary: Accept-Encoding
Content-Type: application/javascript
Content-Length: 674
Keep-Alive: timeout=5, max=99
Connection: Keep-Alive
....

What can it be?

Try turning the output handler off in PHP and see if things work correctly.

I found this comment on the php manual page that is probably what you are running into.

You could try that and if it is the problem, use Apache's SetOutputFilter to gzip the content for you by adding the following to httpd.conf instead of having PHP doing it

<Location />
# Insert filter
SetOutputFilter DEFLATE

# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html

# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4\.0[678] no-gzip

# MSIE masquerades as Netscape, but it is fine
# BrowserMatch \bMSIE !no-gzip !gzip-only-text/html

# NOTE: Due to a bug in mod_setenvif up to Apache 2.0.48
# the above regex won't work. You can use the following
# workaround to get the desired effect:
BrowserMatch \bMSI[E] !no-gzip !gzip-only-text/html

# Don't compress images
SetEnvIfNoCase Request_URI \
\.(?:gif|jpe?g|png)$ no-gzip dont-vary

# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
</Location>