//apache_request_headers — 获取全部 HTTP 请求头信息
$headers = apache_request_headers();

foreach ($headers as $header => $value) {
    echo "$header: $value <br />\n";
}

//get_headers() 返回一个数组，包含有服务器响应一个 HTTP 请求所发送的标头。
$url = 'http://www.example.com';

print_r(get_headers($url));

//如果参数设为 1，则 get_headers() 会解析相应的信息并设定数组的键名。
print_r(get_headers($url, 1));

返回：
Array
(
    [0] => HTTP/1.1 200 OK
    [1] => Date: Sat, 29 May 2004 12:28:13 GMT
    [2] => Server: Apache/1.3.27 (Unix)  (Red-Hat/Linux)
    [3] => Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT
    [4] => ETag: "3f80f-1b6-3e1cb03b"
    [5] => Accept-Ranges: bytes
    [6] => Content-Length: 438
    [7] => Connection: close
    [8] => Content-Type: text/html
)

Array
(
    [0] => HTTP/1.1 200 OK
    [Date] => Sat, 29 May 2004 12:28:14 GMT
    [Server] => Apache/1.3.27 (Unix)  (Red-Hat/Linux)
    [Last-Modified] => Wed, 08 Jan 2003 23:11:55 GMT
    [ETag] => "3f80f-1b6-3e1cb03b"
    [Accept-Ranges] => bytes
    [Content-Length] => 438
    [Connection] => close
    [Content-Type] => text/html
)