PHP image generation & caching

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I hope you can help - I'm a bit new to PHP.

My basic problem is with a page (HTML) which is intended to display an
image slideshow. On this page there are a number of links to click
(one for each slideshow topic) and a "TV screen" to view the results.

The images are stored on the server as (big) JPG files - and when they
are downloaded for the slideshow, I resize them with a PHP script
"im.php" :

$filename = $_GET['filename'];
header('Content-type: image/jpeg');
..... open $filename & do the resizing
imagejpeg( ...

After a link is selected, I preload the set of images with Javascript
by doing :

im[i].src = "php/im.php?filename=" + getFName(i); // for each in the

where getFname(i) returns the i'th image in the set.

I then start the slideshow (I've got things in the Javascript to make
sure all images are preloaded before starting)

Now, my problem is that the browser doesn't seem to be caching the
images and keeps making server requests during the slidehow (which
rather defeats the idea of pre-loading)

If I "cheat" and replace the preloading with :

im[i] = getFName(i);

it works fine (i.e I can switch the server off & it continues to show
the slideshow).

This happens with all browsers I've tried (IE6, Firefox, Netscape)

Hope all this makes sense & would appreciate your expert advice.

Mr WZ Boson

Re: PHP image generation & caching

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Get a packetsniffer (like ethereal) and study the differences in request
and response headers. Also a working example would be nice so "we" will
not have to code (and maybe do something completly different) to see
what you are actually talking about.

Re: PHP image generation & caching

I would also guess that browsers by now have learned not to cache
dynamic pages and when they see im.php?something they will either
detect it as dynamic page or as unknown file type and not cache it
perhaps. When you put the real filename however, they have no problem
saving the .jpg image in their cache. Another thing to look at is if
you disable caching somewhere in your im.php file via header() function
calls or something like that.

Re: PHP image generation & caching

URLs with a querystring (i.e., ones that contain a "?") do not get cached by
the browser.

- Robert

Re: PHP image generation & caching

Robert Tweed wrote:

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That depends on the caching directives.  Responses to such
requests are cacheable if they include explicit expiration
times.  See HTTP/1.1 sec. 13.9.


Re: PHP image generation & caching

Quoted text here. Click to load it

Well, if you want the browser to cache the images, then the images really
aren't dynamic now, are they? So why border serving them dynamically using a
PHP script? I mean you are making your server process the same image over
and over again. Just save the smaller versions of the images in a separate
folder and let the web server handle them.

Re: PHP image generation & caching

true, yet not always applicable. What if he changes the images very
often? It's much easier just to drop the new images in, rather than
making two copies of each image, etc. :) After all that's what
computers are for - to do the dirty and repetative job for us.
Of course on the other hand he can have a script to do the resizing
also automatically ... but that "if" situations are endless :)

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