set_header invalid?

[eluser]Unknown[/eluser]
hi,all

in my api.php:
<?php
......
$this->output->set_header("Content-Type: application/json; charset=UTF-8");
......
?>
When I visited, the browser returns response_header is "Content-Type:text/html",why?

My environment:
Nginx:1.0.4
PHP-FastCGI:5.2.14