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

tldr; both HTTP/1.1 and HTTP/2 headers are case-insensitive.

According to RFC 7230 (HTTP/1.1):

Each header field consists of a case-insensitive field name followed by a colon (":"), optional leading whitespace, the field value, and optional trailing whitespace.

https://tools.ietf.org/html/rfc7230#section-3.2

Also, RFC 7540 (HTTP/2):

Just as in HTTP/1.x, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion.

https://tools.ietf.org/html/rfc7540#section-8.1.2