Are HTTP headers case-sensitive?

Question

In a blog post I use the following PHP to set the content-type of a response:

header(\'content-type: application/json; charset=utf-8\');

I

Answer 1

header('Content-type: image/png') did not work with PHP 5.5 serving IE11, as in the image stream was shown as text

header('Content-Type: image/png') worked, as in the image appeared as an image

Only difference is the capital 'T'.