How to force browser to cache on-demand generated images?

Question

An URL such as http://mydomain.com/text/SGVsbG8gd29ybGQh/params/Y29sb3I9I2Y5NzMwNixmb250LWZhbWlseT1Db21pYyBTYW5zLGZvbnQtc2l6ZT0yMnB4LGZvbnQtd2VpZ2h0PWJvbGQsZm9udC1zdHlsZT1ub3JtYWwsdGV4dC1kZWNvcmF0aW9uPW5vbmUsdGV4dC1hbGlnbj1sZWZ0/96 calls on a PHP script that generates an image based on the encoded information contained within the URL. The information encode is the actual text, plus some formatting options (e.g. color, font-weight etc.).

The first time this URL is called, the image is generated, saved to disk and then returned as the response. Starting with the second call to the same URL, the image is loaded from disk and output to the client instead of being generated all over again.

How can I force the browser to treat such a URL as any other path to a stored image file and cache it, so that the next time the URL is called, the browser will load the image from its cache?

Btw, the code I have in the controller (CodeIgniter-based) is as follows:

public function addText($text, $params, $resolution=96) {
        $this->load->model("textimage_model");
        $image = $this->textimage_model->getImage($text, $params, $resolution);

        // Prepare the response headers and output the image to the client
        header("Content-type: image/png");
        header("Content-disposition: inline; filename=mytext.png");

        if (is_string($image)) {
            readfile($image);
        } elseif (is_resource($image)) {
            imagepng($image);
            imagedestroy($image);
        } else {
            //
        }

    }

Answer 1

Send caching headers.

$seconds_to_cache = 60 * 60 * 24; // 1 day
header("Pragma: cache");
header("Cache-Control: max-age=" . $seconds_to_cache);

Since your caching mechanism seems to be a fingerprint you can have the browser cache it forever.