Description:
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From manual page: http://www.php.net/function.header%23Parameters
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The example for a 404 header assumes you are using http/1.0 it should be 
header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found") also, for the 404 example and  
the location example should at lest show you how to use the http_response_code 
maybe in a example #3

Test script:
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<?php

header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found", null, 404);