为什么PHP header（）不起作用？

<?php
    header('Content-type: application/xml');
    echo "test";
?>

I use this code and its not working in MAMP and the response headers are:

Connection:keep-alive
Content-Encoding:gzip
Content-Type:text/html; charset=UTF-8
Date:Wed, 15 Mar 2017 15:12:45 GMT
Server:nginx/1.11.4
Transfer-Encoding:chunked
X-Powered-By:PHP/7.1.1

Try this instead:

<?php
    header('Content-type: application/xml');
    echo "<example>hello, world!</example>";

Gives you:

Connection:Keep-Alive
Content-Type:application/xml
Date:Wed, 15 Mar 2017 15:12:45 GMT
Server:nginx/1.11.4
Transfer-Encoding:chunked
X-Powered-By:PHP/7.1.1

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