i found that 'php' is also supported.

<?php
$solr_server = array
(
    'hostname'     => $solr_hostname,
    'port'         => $solr_port,
    'path'         => $solr_path,
);
$solr_client = new SolrClient($solr_server);

$solr_response_writer = 'php'; // "wt"
$solr_client->setResponseWriter($solr_response_writer);

$solr_response = new SolrObject();

$solr_query = new SolrQuery();
$solr_query->setQuery($solr_query_string); // "q"

try {
    $query_response = $solr_client->query($solr_query);
    $solr_response = $query_response->getResponse();
    return '';
} catch (Exception $e) {
    return ($e);
}
?>

php_error.log keeps on saying:

[12-Aug-2015 12:34:56 Asia/Hong_Kong] PHP Warning:  SolrClient::setResponseWriter(): Unsupported response writer php. This value will be ignored in C:\www\...\solr.php on line 21

but the returned data is good as expected:

<?php
$html .= '<table cellSpacing="1" cellPadding="1" border="1">';
$html .= '<tr>';
$html .= '<td>id</td>';
$html .= '<td>title</td>';
$html .= '<td>score</td>';
$html .= '</tr>';

for ($i_solr = 0; $i_solr <= ($solr_rows - 1); $i_solr++) {
    $html .= '<tr>';
    $html .= '<td>' . $solr_response['response']['docs'][$i_solr]['id'] . '</td>';
    $html .= '<td>' . $solr_response['response']['docs'][$i_solr]['title'][0] . '</td>';
    $html .= '<td>' . $solr_response['response']['docs'][$i_solr]['score'] . '</td>';
    $html .= '</tr>';
}
$html .= '</table>';
echo $html;
?>