This function doesn't exist on lighttpd, so I wrote this little function to emulate it:

<?php

if (!function_exists('apache_response_headers')) {
    function apache_response_headers () {
        $arh = array();
        $headers = headers_list();
        foreach ($headers as $header) {
            $header = explode(":", $header);
            $arh[array_shift($header)] = trim(implode(":", $header));
        }
        return $arh;
    }
}

?>

If apache_response_headers() returns an empty array, try calling flush() before and it'll get filled.

Addendum: The code

<pre>
<?php
  print_r(apache_request_headers());
?>
</pre>

gives me

 Array
 (
    [X-Powered-By] => PHP/5.1.6
 )

But the code
<pre>
<?php
  ob_end_flush();
  print_r(apache_request_headers());
?>
</pre>

Results in

 Array
 (
    [Content-Location] => phpinfo.de.php
    [Vary] => negotiate
    [TCN] => choice
    [X-Powered-By] => PHP/5.1.6
    [Keep-Alive] => timeout=15, max=96
    [Connection] => Keep-Alive
    [Transfer-Encoding] => chunked
    [Content-Type] => text/html
    [Content-Language] => de
 )

I get  "Call to undefined function apache_response_headers()" when using php-cli so be aware of lack of this function CLI.
In my case I was trying to test my app via phpunit (command line tool of course ;)) if it sends proper headers.

For apache_response_headers() working good, you need to set  up output_buffering = Off in php.ini

I can confirm athlet's experience with apache_response_headers() using PHP 5.1.6. When the resulting array is empty or only contains "X-Powered-By" instead of the full list of values, you'll need to switch off output_buffering _before_ the script has been executed, for example with the following directive in .htaccess:

php_value "output_buffering" "0"

it _doesn't_ work when you do a you ini_set('output_buffering', 0); in your script. Not sure why that is so - it is not a bug in PHP, but probably has to do with how Apache populates the request values.