一、认识HTTP

    HTTP协议历史及设计思路（点击浏览）

二、获取浏览器HTTP请求header信息

    1. Apach服务器下可以直接使用 PHP自带函数获取客户端HTTP请求头信息
 

/*
作用：获取客户端HTTP请求所有头信息（header）
参数：无。
返回：HTTP请求所有头信息数组
*/
getallheaders()

 实例：

<?php
 var_dump(getallheaders());
 
//==>输出如下：

array(12) {
  ["Content-Type"] => string(0) ""
  ["Content-Length"] => string(1) "0"
  ["X-Original-Url"] => string(21) "/Home/Other/getHeader"
  ["Upgrade-Insecure-Requests"] => string(1) "1"
  ["User-Agent"] => string(114) "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.119 Safari/537.36"
  ["Host"] => string(17) "www.example.com"
  ["Cookie"] => string(36) "PHPSESSID=7rjh2uomb8477dggmr85bg9067"
  ["Accept-Language"] => string(14) "zh-CN,zh;q=0.9"
  ["Accept-Encoding"] => string(13) "gzip, deflate"
  ["Accept"] => string(85) "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8"
  ["Connection"] => string(10) "keep-alive"
  ["Cache-Control"] => string(9) "max-age=0"
}

2.  非apach环境下，可根据服务器$_SERVER信息获取HTTP请求的header信息，设计函数：

function getHeader() {
    $headers = array(); 
    foreach ($_SERVER as $key => $value) {
        if ('HTTP_' == substr($key, 0, 5)) { 
            $headers[str_replace('_', '-', substr($key, 5))] = $value; 
        }
        if (isset($_SERVER['PHP_AUTH_DIGEST'])) { 
            $header['AUTHORIZATION'] = $_SERVER['PHP_AUTH_DIGEST']; 
        } elseif (isset($_SERVER['PHP_AUTH_USER']) && isset($_SERVER['PHP_AUTH_PW'])) { 
            $header['AUTHORIZATION'] = base64_encode($_SERVER['PHP_AUTH_USER'] . ':' . $_SERVER['PHP_AUTH_PW']); 
        } 
        if (isset($_SERVER['CONTENT_LENGTH'])) { 
            $header['CONTENT-LENGTH'] = $_SERVER['CONTENT_LENGTH']; 
        } 
        if (isset($_SERVER['CONTENT_TYPE'])) { 
            $header['CONTENT-TYPE'] = $_SERVER['CONTENT_TYPE']; 
        }
    }
    return $headers;
}

实例：

<?php
  var_dump(getHeader());

 // ==>输出 如下：

array(11) {
  ["X-ORIGINAL-URL"] => string(21) "/Home/Other/getHeader"
  ["UPGRADE-INSECURE-REQUESTS"] => string(1) "1"
  ["USER-AGENT"] => string(114) "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.119 Safari/537.36"
  ["HOST"] => string(17) "www.example.com"
  ["COOKIE"] => string(36) "PHPSESSID=7rjh2uomb8477dggmr85bg9067"
  ["ACCEPT-LANGUAGE"] => string(14) "zh-CN,zh;q=0.9"
  ["ACCEPT-ENCODING"] => string(13) "gzip, deflate"
  ["ACCEPT"] => string(85) "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8"
  ["CONTENT-LENGTH"] => string(1) "0"
  ["CONNECTION"] => string(10) "keep-alive"
  ["CACHE-CONTROL"] => string(9) "max-age=0"
}

三、获取服务器HTTP响应header信息

/*
  作用：获取服务器响应一个 HTTP 请求所发送的所有标头
  参数：
       url：目标 URL；
       format：如果将可选的 format 参数设为 1，则 get_headers() 会解析相应的信息并设定数组的键名。
  返回：返回包含有服务器响应一个 HTTP 请求所发送标头的索引或关联数组，如果失败则返回 FALSE。
*/
get_headers ( string $url [,int $format = 0 ] )

实例：

<?php
  $url = 'http://www.example.com';
  print_r(get_headers($url));
  print_r(get_headers($url, 1));
?>
 // ==>输出 如下：

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
)

四，参考资料

1.  https://www.yiibai.com/manual/php/function.get-headers.html

2.  https://www.oschina.net/question/54100_38761

3.  https://blog.csdn.net/xuezhiwu001/article/details/61203045