You'll need to set XML headers if you want to output the file directly:
Using the Codeigniter Output class:
$xml = $this->dbutil->xml_from_result($query, $config);
$this->output->set_content_type('text/xml');
$this->output->set_output($xml);
Or you can use plain PHP to set the headers:
header('Content-type: text/xml');
echo $this->dbutil->xml_from_result($query, $config);
Or you can use the CI download helper:
$xml = $this->dbutil->xml_from_result($query, $config);
$this->load->helper('download');
force_download('myfile.xml', $xml);
Or write it to a file with the file helper:
$xml = $this->dbutil->xml_from_result($query, $config);
$this->load->helper('file');
$file_name = '/path/to/myfile.xml';
write_file($file_name, $xml);
// Optionally redirect to the file you (hopefully) just created
redirect($file_name);
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