LightPress

One of the benefits we might get from the new architecture I'm testing is a much more integrated and simpler feed management, so while adapting the current code to the new class layout I took the occasion to revise or support for Conditional Get.

I had already adapted Simon Willison's code to work around a bug in PHP's

formats (fixed in 4.3.11), but the code was a bit too complex for my tastes, and re-reading the relevant parts of the HTTP/1.1 specification, I noticed that full compliance requires supporting three different date formats, all in GMT:

Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123


Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036


Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format

All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception.

HTTP/1.1 clients and servers that parse the date value MUST accept all three formats (for compatibility with HTTP/1.0), though they MUST only generate the RFC 1123 format for representing HTTP-date values in header fields.

So I rewrote the relevant part of our code from scratch, and got the following function. It's still untested (I know, I know, I should write unit tests first...) Please let me know about any problems/missing features/errors.

Update

I kept wondering if checking the dates as strings is enough, and finally decided to check RFC2616 again and found the following sentence in section 14.25

When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header field whenever possible.

So maybe dates as strings are enough, even though proper date checking would be better.
<br /> function doConditionalGet($tstamp) {<br /> /*<br /> * A PHP implementation of conditional get for LightPress, see also<br /> * http://fishbowl.pastiche.org/archives/001132.html<br /> * http://simon.incutio.com/archive/2003/04/23/conditionalGet<br /> *<br /> */</p> <p>// ETag is any quoted string<br /> $etag = '"'. $tstamp .'"';</p> <p>// RFC1123 date, see http://bugs.php.net/bug.php?id=31842<br /> if (version_compare(PHP_VERSION, "4.3.11", ">="))<br /> $format = 'r';<br /> else<br /> $format = 'D, d M Y H:i:s O';</p> <p>$rfc1123 = substr(gmdate('r', $tstamp), 0, -5) . 'GMT';</p> <p>// RFC1036 date<br /> $rfc1036 = gmdate('l, d-M-y H:i:s ', $tstamp) . 'GMT';</p> <p>// asctime<br /> $ctime = gmdate('D M j H:i:s', $tstamp);</p> <p>// Send the headers<br /> header("Last-Modified: $rfc1123");<br /> header("ETag: $etag");</p> <p>// See if the client has provided the required headers</p> <p>$if_modified_since = $if_none_match = false;</p> <p>if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))<br /> $if_modified_since = stripslashes($_SERVER['HTTP_IF_MODIFIED_SINCE']);</p> <p>if(isset($_SERVER['HTTP_IF_NONE_MATCH']))<br /> $if_none_match = stripslashes($_SERVER['HTTP_IF_NONE_MATCH']);</p> <p>if (!$if_modified_since && !$if_none_match) {<br /> // both are missing<br /> return $rfc1123;<br /> }</p> <p>// At least one of the headers is there - check them</p> <p>// check etag if it's there and there's no if-modified-since<br /> if ($if_none_match) {<br /> if ($if_none_match != $etag) {<br /> // etag is there but doesn't match<br /> return $rfc1123;<br /> }<br /> if (!$if_modified_since && ($if_none_match == $etag)) {<br /> header('HTTP/1.0 304 Not Modified');<br /> exit;<br /> }<br /> }</p> <p>if ($if_modified_since) {<br /> // check if-modified-since</p> <p>foreach (array($rfc1123, $rfc1036, $ctime) as $d) {<br /> if ($d == $if_modified_since) {<br /> // Nothing has changed since their last request - serve a 304 and exit<br /> header('HTTP/1.0 304 Not Modified');<br /> exit;<br /> }<br /> }<br /> }</p> <p>// return $rfc1123 as it may be useful later, eg 'lastBuildDate' for RSS2<br /> return $rfc1123;</p> <p>}<br />