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

officially, headers are case insensitive, however, it is common practice to capitalize the first letter of every word.
but, because it is common practice, certain programs like IE assume the headers are capitalized.
so while the docs say the are case insensitive, bad programmers have basically changed the docs.