HTTP if-none-match and if-modified-since and 304 clarification in PHP

Question

My question is about how to reply a HTTP 304 "Not Modified" when I receive both if-none-match and if-modified-since from a proxy/client request.

From RFC 2616 secttion 14.26 ( http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26 ):

If none of the entity tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity tags match, then the server MUST NOT return a 304 (Not Modified) response.

I am not sure to understand this statement...

1. "If none of the entity tags match" in PHP do they speak of $_SERVER['HTTP_IF_NONE_MATCH'] vs. my ETags that I sent earlier?
2. If I understand correctly this statement, as soon as none of the ETags listed in $_SERVER['HTTP_IF_NONE_MATCH'] match my ETags, I stop all verifications and serve the page normally.

Anyone can translate this RFC part in pseudo-code (or PHP code) and/or answer my 2 points above?

EDIT 1: Thank you St.Woland for your answer. Can you (or anyone else) tell me if I'm correct on these 6 points:

1. The format of $_SERVER['HTTP_IF_NONE_MATCH'] can be either:

   a)If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"

   b) If-None-Match: "xyzzy"

   and NOT:

   c) If-None-Match: "xyzzy, r2d2xxxx, c3piozzzz"

2. If !array_key_exists('HTTP_IF_NONE_MATCH', $_SERVER), anyTagMatched() returns NULL.

3. As soon as an ETag in $_SERVER['HTTP_IF_NONE_MATCH'] match my document ETag, anyTagMatched() returns TRUE.

4. If none of the Etags in $_SERVER['HTTP_IF_NONE_MATCH'] are matching my document ETag, anyTagMatched() returns FALSE.

5. If $_SERVER['HTTP_IF_MODIFIED_SINCE'] is set and matches my document "last modified" date isExpired() returns FALSE, otherwise return TRUE.

6. As soon as anyTagMatched() returns TRUE, I issue a 304. If anyTagMatched() returned NULL and isExpired() returned FALSE I can issue a 304. In any other situation I serve my page as normal (I also issue the up-to-date Last-Modified and ETag headers).

Answer 1

This should be put in the end (moved for better look).

$anyTagMatched = anyTagMatched() ;
if( $anyTagMatched || ( ( null === $anyTagMatched ) && !isExpired() ) ) {
    notModified() ;
}
// Output content

Pseudocode (review needed):

<?php

/**
 * Calculates eTag for the current resource.
 */
function calculateTag() {
}

/**
 * Gets date of the most recent change.
 */
function lastChanged() {
}

/**
 * TRUE if any tag matched
 * FALSE if none matched
 * NULL if header is not specified
 */
function anyTagMatched() {
    $if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ?
        stripslashes($_SERVER['HTTP_IF_NONE_MATCH']) : 
        false ;

    if( false !== $if_none_match ) {
        $tags = split( ", ", $if_none_match ) ;
        $myTag = calculateTag() ;
        foreach( $tags as $tag ) {
            if( $tag == $myTag ) return true ;
        }
        return false ;
    }
    return null ;
}

function isExpired() {
    $if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ?
        stripslashes($_SERVER['HTTP_IF_MODIFIED_SINCE']) :
        false;

    if( false !== $if_modified_since ) {
        // Compare time here; pseudocode.
        return ( $if_modified_since < lastChanged() ) ;
    }

    return true ;
}

function notModified() {
    header('HTTP/1.0 304 Not Modified');
    exit ;
}

Main answer here.