[php] 解析JSON字符串

$json_str = '{"thumb":"","photo":[{"url":"557a3d2184db0.png","alt":"SafeBoot\u6ce8\u518c\u8868"}]}';  // json字符串
$obj = json_decode($json_str);
$photo = $obj->photo;
$url = $photo[0]->url;  // echo url; //557a3d2184db0.png