Description:
------------
If PHP (5.3.0) is running as an (Apache 2) module, it currently sets no_local_copy to 1 on the response it sends to Apache (sapi/apache2handler/sapi_apache2.c:463). It looks like this flag was set to disallow Apache from erroneously creating its own "304 Not Modified" responses based on the ETag or Last-Modified-Date of the PHP scripts' sourcecode itself, which would result in stale pages being served if the scripts' output changes over time.

But there's a serious side effect of setting this flag in combination with Apache's mod_cache. If the browser makes a conditional request for a cached PHP document, but the document is expired in the cache, mod_cache correctly passes on the conditional request to PHP. If the PHP script responds with a "304 Not Modified" code, mod_cache should generate a 304 response for the browser. But due to no_local_copy, Apache is denied from creating a 304 code in response to a request for a PHP document. This forces it to resend the (still valid) body of the PHP document from the cache with a 200 code.

But setting "no_local_copy=1" is not needed anyway. Just below the r->no_local_copy=1 line in sapi_apache2.c is a series of calls to apr_table_unset which remove any headers that Apache might have generated based on the PHP source itself and could be using to accept conditional requests. Starting at line 468 in php_apache_request_ctor, we have:

apr_table_unset(r->headers_out, "Content-Length");
apr_table_unset(r->headers_out, "Last-Modified");
apr_table_unset(r->headers_out, "Expires");
apr_table_unset(r->headers_out, "ETag");

It seems to me that removing the r->no_local_copy=1 will therefore not result in erroneous "304 Not Modified" responses being sent by Apache for PHP scripts.

At the moment if you request a mod_cache'd PHP page which itself sends no special caching directives (e.g. an empty script), the reply from the server is (trimmed to include only cache-relevant directives):

Status=OK - 200
Date=Sun, 26 Jul 2009 10:07:58 GMT

PHP has correctly suppressed the generation of Last-Modified-Date and ETag headers based on the source of the script itself. No conditional request is possible and you won't get a stale page.

Now, if you request a PHP document that does set "ETag", such as the attached code "index.php", the response from the server is:

Status=OK - 200
Date=Sun, 26 Jul 2009 10:11:02 GMT
Etag="ComputedETag"
Expires=Tue, 25 Aug 2009 10:11:02 GMT

And the error log shows that the script correctly returned a 200 response code to Apache. Now if you press "refresh" in Firefox, the browser sends this request:

If-None-Match="ComputedETag"
Cache-Control=max-age=0

This is a conditional get which will also result in Apache revalidating its cache (since max-age=0). So Apache passes the conditional request onto PHP, and PHP sends back a 304 Not Modified response. But due to no_local_copy, mod_cache cannot send a 304, it responds to the browser with:

Status=OK - 200
Date=Sun, 26 Jul 2009 10:11:35 GMT
Etag="ComputedETag"
Expires=Tue, 25 Aug 2009 10:11:35 GMT

So, I removed the line that sets no_local_copy in my PHP. This had no impact on the way that the empty PHP document that sets no cache directives was served, Apache never served erroneous 304 responses because it never saw ETag or Last-Modified headers based on the PHP source. But the ETag-conditional request for index.php by the browser now gives the correct 304 response code:

Status=Not Modified - 304
Date=Sun, 26 Jul 2009 10:16:23 GMT
Etag="ComputedETag1"
Expires=Tue, 25 Aug 2009 10:16:23 GMT

Reproduce code:
---------------
<?php

$etag="\"ComputedETag\"";

header("Etag: $etag");
//Expires ages away
header("Expires: " . gmdate("D, d M Y H:i:s", time()
    + 60 * 60 * 24 * 30) . " GMT");

if (isset($_SERVER['HTTP_IF_NONE_MATCH']) &&
    $_SERVER['HTTP_IF_NONE_MATCH'] == $etag) {

    /* At a users' request, the cache has been bypassed, but the
     * document is still the same. Avoid costly response generation
     * and waste of bandwidth by just sending not-modified.
     * (it is illegal to send a response-body by the HTTP spec).
     */
    header('HTTP/1.0 304 Not Modified');
    
    error_log(date('r')." - Response: 304 Not Modified");
    exit(); //Don't generate or send the body
}   

error_log(date('r')." - Response: 200. Generated document.");

echo "Document body goes here";

?>