Handling If-modified-since header in a PHP-script

Question

I have a PHP script which is called with an ?img= parameter.

The value for that parameter is an (urlencoded) URL of an image.

My script checks, if that image is already stored at my server.

If not - it downloads it. After that it optionally resizes the image and sends it to STDOUT, i.e. back to the requesting browser, prepended with Content-Type and Last-modified headers:

Connection:close
Content-Type:image/jpeg
Date:Fri, 01 Jun 2012 08:28:30 GMT
Last-Modified:Fri, 01 Jun 2012 08:02:44 GMT
Server:Apache/2.2.15 (CentOS)
Transfer-Encoding:chunked
X-Powered-By:PHP/5.3.3

This is needed to workaround some crossdomain issues and works well for me since over a year:

However I'd like to add functionality to handle the incoming If-Modified-since header - to send a Not Modified 304 response.

My questions are:

1) Is that even possible in PHP, when run in Apache?

2) How to handle (i.e. parse and produce) the dates best in PHP here?

Bonus question) How to add a Content-Length header for the resized images?

My code is below (I've omitted the CURL-downloading part):

<?php

define('CACHE_DIR', '/var/www/cached_avatars/');

$img    = urldecode($_GET['img']);
$cached = CACHE_DIR . md5($img);

# omitted downloading part for brevity

$readfh = fopen($cached, 'rb');
if ($readfh) {
        flock($readfh, LOCK_SH);

        $size = getimagesize($cached);
        $w    = $size[0];
        $h    = $size[1];
        $type = $size[2];
        $mime = $size['mime'];

        # find the downscale factor to fit image into $maxw x $maxh
        $scale = max($w / $maxw, $h / $maxh);

        header('Content-Type: ' . $size['mime']);
        header('Last-Modified: ' . gmdate('D, d M Y H:i:s T', filemtime($cached)));

        $length = filesize($cached);
        $buf = fread($readfh, $length);
        fclose($readfh);

        # the image is smaller than $maxw x $maxh, do not scale up
        if ($scale <= 1) {
                header('Content-Length: ' . $length);
                print($buf);
                return;
        }

        $tw = $w / $scale;
        $th = $h / $scale;
        $image = imagecreatefromstring($buf);
        $thumb = imagecreatetruecolor($tw, $th);
        imagecopyresampled($thumb, $image, 0, 0, 0, 0, $tw, $th, $w, $h);
        imagedestroy($image);

        # How to add Content-Length here, after image resizing?

        if (IMAGETYPE_JPEG == $type)
                imagejpeg($thumb, null, 75);
        else if (IMAGETYPE_PNG == $type)
                imagepng($thumb, null, 9);
        else if (IMAGETYPE_GIF == $type)
                imagegif($thumb, null);

        imagedestroy($thumb);
}

?>

Auberta

Answer 1

This is definitely possible in PHP!

When the browser checks if there were modifications, it sends an If-Modified-Since header; in PHP this value would be set inside $_SERVER['HTTP_IF_MODIFIED_SINCE'].

To decode the date/time value (encoded using rfc822 I believe), you can just use strtotime(), so:

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && 
    strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= filemtime($localFileName))
{
    header('HTTP/1.0 304 Not Modified');
    exit;
}

Explanation: if the If-Modified-Since header is sent by the browser AND the date/time is at least the modified date of the file you're serving, you write the "304 Not Modified" header and stop.

Otherwise, the script continues as per normal.

Merchant

If it's helpful, I wrote this parser for HTTP dates that validates, and covers all 3 formats: github.com/evert/SabreDAV/blob/master/lib/Sabre/HTTP/… – Lingerfelt

Thank you all, this works and I've opted for comparing the strings instead of parsing the dates - this seems to work well in my case: $mod = gmdate('D, d M Y H:i:s T', filemtime($cached)); if (@$_SERVER['HTTP_IF_MODIFIED_SINCE'] == $mod) { header('HTTP/1.0 304 Not Modified'); exit; } – Auberta

@Merchant How Ican do the same in bash or C99? – Gillead

Answer 2

I recently had to use this feature (serving image via PHP) in order to make images visible only for registered users. Jack's code was helpful, but I had to do a few hacks for it to work perfectly. Thought I should share this one.

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && 
    strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= filemtime($path_to_image))
{
    header('HTTP/1.0 304 Not Modified');
    header("Cache-Control: max-age=12096000, public");
    header("Expires: Sat, 26 Jul 2015 05:00:00 GMT");
    header("Pragma: cache");
    exit;
}else{
    header("Content-type: image/jpeg");
    header("Cache-Control: max-age=12096000, public");
    header("Expires: Sat, 26 Jul 2015 05:00:00 GMT");
    header("Pragma: cache");
    echo file_get_contents($path_to_image);
}

In short, the script returns Not Modified if it's a browser request HTTP_IF_MODIFIED_SINCE. Otherwise, the image is served with appropriate headers and expiry dates.

Sweetheart

don't read your whole image file into memory, stream it to the browser with readfile($path_to_image) – Pippo