Source

File: wp-includes/IXR/class-IXR-server.php

$this->output($error->getXml());
    }

    function output($xml)
    {
        $charset = function_exists('get_option') ? get_option('blog_charset') : '';
        if ($charset)
            $xml = '<?xml version="1.0" encoding="'.$charset.'"?>'."\n".$xml;