... HTTP 101 ...
A document transmitted via the HTTP protocol will have two sections - a header and a body. The way modern browsers work, you never see the header, but they are there. These are the response headers for Google.
Date: Fri, 24 Feb 2012 13:59:47 GMT
Cache-Control: private, max-age=0
Content-Type: text/html; charset=UTF-8
X-XSS-Protection: 1; mode=block
After this information comes the MIME encoded body of the document. If it's text it will be relatively readable.
PHP can control, through the header function, the contents of any of these lines. This allows you to modify the response code, caching and so on. Whatever you don't populate your webserver program populates for you according to its own settings.
meta http-equiv tags will, in theory, override these properties. But it's more efficient to pass the correct desired value in the header in the first place. Also the content-type header cannot be changed after rendering of the document has started, so http-equiv="Content-Type" is useless and will be ignored. The same applies to the Content-Encoding and Content-Length properties. Meta http-equiv tags are primarily used for setting specific caching rules in otherwise static html documents, and they are quite effective in that role.