PHP: Malformed header

Could someone explain what is wrong with this PHP code? (PHP Version 5.3.5)

<?php 
    header('Bad Request', true, 400);
    exit;

I get a status 开发者_C百科500 instead of 400. And in the apache error log I get

malformed header from script. Bad header=Bad Request: listener.php

What am I doing wrong here? Have I misunderstood the docs?

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you have to write:

header('HTTP/1.0 400 Bad Request', true, 400);