Description:
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The header, headers_list, headers_sent and possibly other functions use the term "header" incorrectly (although this is a very common mistake).  See RFC 2616 sections 4.1 and 4.2 for the complete explanation.

Essentially, an HTTP response consists of a status-line, message-header and message-body.  The individual entities in the header are called "header fields", not "headers" as the documentation calls them.

I realize this is a pedantic issue, but that doesn't mean it shouldn't be fixed.  The PHP manual is generally excellent.