public static function getHeadersToSign($headers, $headersToSign)
{

//value被trim后为空串的header不参与签名
$filter_empty = function($v) {
return trim((string) $v) !== '';
};
$headers = array_filter($headers, $filter_empty);

//处理headers的key：去掉前后的空白并转化成小写
$trim_and_lower = function($str){
return strtolower(trim($str));
};
$temp = array();
$process_keys = function($k, $v) use(&$temp, $trim_and_lower) {
$temp[$trim_and_lower($k)] = $v;
};