What case to use in HTTP headers?

Question

PHP

header('Content-Type: text/html; charset=UTF-8');
header('Content-Type: text/html; charset=utf-8');
header('Content-type: text/html; charset=utf-8');
header('content-type: text/html; charset=utf-8');

Answer 1

According to RFC 2616 , "Hypertext Transfer Protocol - HTTP/1.1", §4.2, "Message Headers": header name is case insensitive