Flickr照片搜索API没有返回任何内容

我尝试使用cURL和flickr API进行flickr搜索。当尝试打印响应时，它会打印"1"。我的代码出了什么问题？

$params = array(
    'api_key'   => 'b838e46f6e8eada6a62fac7e2b25ffcc',
    'method'    => 'flickr.photos.search',
    'format'    => 'php_serial',
    'text'  =>'cars'
);
$encoded_params = array();
foreach($params as $k => $v){
    $encoded_params[] = urlencode($k).'='.urlencode($v);
}
$ch = curl_init();
$timeout = 0; // set to zero for no timeout
curl_setopt ($ch, CURLOPT_URL, 'https://api.flickr.com/services/rest/?'.implode('&', $encoded_params));
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, $timeout);
$file_contents = curl_exec($ch);
print_r($file_contents);
curl_close($ch);
$rsp_obj = unserialize($file_contents);
//echo 'https://api.flickr.com/services/rest/?'.implode('&', $encoded_params);
echo print_r($rsp_obj);