.htaccess redirecting to a php file which uses header() will execute twice

Question

I'm generating dynamic images with PHP and using .htaccess to redirect to the PHP-script to use those images in HTML-img-tags, e.g. "/path/something.jpg".

Here's my .htaccess:

RewriteEngine On
RewriteBase /
RewriteRule ^/?([a-z]+)/(.*)$ /?a=$1&b=$2 [L,NC]

Which will have my index.php receive "path" in $_GET['a'] and "something.jpg" in $_GET['b']. This works as expected.

Now, after creating the image with PHP, the output of the image is done like this:

header('Content-type: image/jpg');
header('Content-Disposition: filename="something.jpg"');
echo $binary;
exit;

This also works, I receive an image, the browser displays it.

BUT: When writing a log whenever index.php is called to generate a picture, I see that it's called TWICE for every image that's being created and I can't figure out why. It seems that using header() will make the script start again, because removing header() from the script (and just printing $binary) will result in the script to be executed only once.

The apache-access.log shows the second call as well:

127.0.0.1 - - [18/Sep/2014:12:17:46 +0200] "GET /path/something.jpg HTTP/1.1" 200 19148 "-" "USER_AGENT"
127.0.0.1 - - [18/Sep/2014:12:17:46 +0200] "GET /path/something.jpg HTTP/1.1" 200 19148 "-" "USER_AGENT"

I've been experimenting with .htaccess-settings to prevent this using:

RewriteCond %{ENV:REDIRECT_STATUS} 200
RewriteRule ^ - [L]

in my .htaccess-file (as found here: Apache mod_rewrite REDIRECT_STATUS condition causing directory listing), but to no avail.

I also wonder why this behavior won't result in an endless loop: if .htaccess sends the request to index.php, which executes a header() that will call itself again, why wouldn't this go on forever, but only twice? (on rare occasions even three times, but I wasn't able to reproduce this)

Any hint about why this is happening (and how to fix it!) is very much appreciated. The script works so far, but I'd like to prevent it from generating each picture twice as I'm expecting a lot of serverload once the application is finished.

Answer 1

You didn't specify which is the browser you tested this in, but my guess is that it's Chrome as others have encountered similar issues with it. There's this bug report on the Chromium bug tracker and this answer on StackOverflow.

Try to remove only the Content-Disposition header and see if it works correctly without that set. You can also try to specify INLINE inside your content disposition header like this:

header('Content-Disposition: INLINE; FILENAME= "something.jpg"');

and see if that fixes anything.

If these don't help, try to use a web debugging proxy tool (like Fiddler or Charles) and post the complete request / response headers for both requests.