Imagick saving from browser as a php file

I have a php/imagick script that outputs a png image but when I go to save it from the browser it saves as the file name: index.php I want it to save as a png when you click save as 开发者_C百科image in your browser.

link: http://www.halogamertags.com/signatures/index.php?player=yoinked%20ya

How can I do that?

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header('Content-Disposition: attachment; filename="filename.png"');