TODO: Prefers HTTP/2 Header Style for Both Request and Response

[Taufik Nurrohman]

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

Header field names are strings of ASCII characters that are compared in a case-insensitive fashion. However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2. A request or response containing uppercase header field names MUST be treated as malformed.

Example:

if (send('a@b', 'c@d', 'Title', 'Body as HTML.', [
    'reply-to' => $_POST['email']
])) {
    // Mail sent!
}