Topic awaiting preservation: prevent an image from being cached.

hello,
to fight spambots throughout my website, i use an common technique to prevent forms from being filled out automatically.

a php-script generates a jpg with a number which has to be validated in the form.
the image is generated new each time the page is called.
now the problem is that some browsers (opera) seem to cache this image and therefore the validation process doesnt work anymore.

here is the code i use:

$i=imagecreatetruecolor(50,20);
$r=rand(10000,99999);
imagestring($i, 4, 4, 2, $r, imagecolorallocate($i, 255, 255, 255));
imagejpeg($i,"/antispam.jpg");

then i just use the image-tag to put it on the site.


is there a way to force this image to be reloaded on every pageload without putting the no-cache headers?

If you don't mind to use a bit of JavaScript, you could add a random parameter in the SRC attribute of the image tag, to make sure it reloads it.

Unless the cache of the browser is really hungry, setting the HTTP headers related to cache and expiracy time should be enough to force the browser to not cache the HTTP flux ( the image ).



(Edited by poi on 04-02-2005 13:57)

a random parameter?

like

img src="i.jp" param="123"

i could do this with php too.

I rather thought to : antispamImageGeneratedInPhp.jpg?randomParameter=165781379
but you could also generate a completely random src and catch the error 404 to output your anti spam image.

Using JavaScript you'll be sure that even if the page is cached, the antispam image will be different.




(Edited by poi on 04-02-2005 14:14)

ok thanks!

you mean, that if the page is completely cached, that a php-random parameter would also be cached?

yep

ok, i did it so and it works. thank you poi.

you're welcome

You can also do this with meta tags, of course there are a few things u need to do to get it working in most browsers.

Take a read here: http://www.htmlgoodies.com/beyond/reference/article.php/3472881