头信息很重要，2个人，一个叫服务器（服务端），一个叫浏览器(客户端)，即有2个头信息，请求头信息和返回头信息。
那么如果通过php获取这些头信息，服务端想获取浏览器请求时的头信息，可以用$_SERVER，代码如下：
<?phpfunction GetHeaderInfo(){ foreach ($_SERVER as $name => $value){ $headers[$name] = $value; } return $headers; }var_dump(GetHeaderInfo());?>输出：
array (size=32) 'HTTP_HOST' => string 'localhost' (length=9) 'HTTP_USER_AGENT' => string 'Mozilla/5.0 (Windows NT 5.1; rv:20.0) Gecko/20100101 Firefox/20.0' (length=65) 'HTTP_ACCEPT' => string 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' (length=63) 'HTTP_ACCEPT_LANGUAGE' => string 'zh-cn,zh;q=0.8,en-us;q=0.5,en;q=0.3' (length=35) 'HTTP_ACCEPT_ENCODING' => string 'gzip, deflate' (length=13) 'HTTP_COOKIE' => string 'bdshare_firstime=1365907820098' (length=30) 'HTTP_CONNECTION' => string 'keep-alive' (length=10) 'HTTP_CACHE_CONTROL' => string 'max-age=0' (length=9) 'PATH' => string 'C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;D:\svn\bin;D:\curl\curl-7.19.5;E:\mysql5.5\mysql\bin;C:\Program Files\MySQL\MySQL Server 5.5\bin;C:\Ruby193\bin;C:\Program Files\SinoVoice\jTTS 5.0 Desktop\Bin;D:\php\xampp\php;C:\wamp\bin\php\php5.4.3\php.exe;' (length=266) 'SystemRoot' => string 'C:\WINDOWS' (length=10) 'COMSPEC' => string 'C:\WINDOWS\system32\cmd.exe' (length=27) 'PATHEXT' => string '.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH' (length=48) 'WINDIR' => string 'C:\WINDOWS' (length=10) 'SERVER_SIGNATURE' => string '' (length=0) 'SERVER_SOFTWARE' => string 'Apache/2.2.22 (Win32) PHP/5.4.3' (length=31) 'SERVER_NAME' => string 'localhost' (length=9) 'SERVER_ADDR' => string '127.0.0.1' (length=9) 'SERVER_PORT' => string '80' (length=2) 'REMOTE_ADDR' => string '127.0.0.1' (length=9) 'DOCUMENT_ROOT' => string 'C:/wamp/www/' (length=12) 'SERVER_ADMIN' => string 'admin@localhost' (length=15) 'SCRIPT_FILENAME' => string 'C:/wamp/www/1.php' (length=17) 'REMOTE_PORT' => string '2483' (length=4) 'GATEWAY_INTERFACE' => string 'CGI/1.1' (length=7) 'SERVER_PROTOCOL' => string 'HTTP/1.1' (length=8) 'REQUEST_METHOD' => string 'GET' (length=3) 'QUERY_STRING' => string '' (length=0) 'REQUEST_URI' => string '/1.php' (length=6) 'SCRIPT_NAME' => string '/1.php' (length=6) 'PHP_SELF' => string '/1.php' (length=6) 'REQUEST_TIME_FLOAT' => float 1366034246.187 'REQUEST_TIME' => int 1366034246
可以看到不仅输出客户端的一些信息，也输出了服务端的一些信息。针对这些信息具体作用，这里不多介绍了，请查看http://www.cnblogs.com/IAmBetter/archive/2013/04/11/3014796.html
其实针对apache服务器可以使用现成的函数：getallheaders()，用这个函数的好处不多说了，直接看返回值吧
array (size=8) 'Host' => string 'localhost' (length=9) 'User-Agent' => string 'Mozilla/5.0 (Windows NT 5.1; rv:20.0) Gecko/20100101 Firefox/20.0' (length=65) 'Accept' => string 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' (length=63) 'Accept-Language' => string 'zh-cn,zh;q=0.8,en-us;q=0.5,en;q=0.3' (length=35) 'Accept-Encoding' => string 'gzip, deflate' (length=13) 'Cookie' => string 'bdshare_firstime=1365907820098' (length=30) 'Connection' => string 'keep-alive' (length=10) 'Cache-Control' => string 'max-age=0' (length=9)
完全没有类似于HTTP_这样的参数，还需要去处理等问题。
知道如何获取请求过来的头信息了，那么也应该知道如何设置头信息，比如：
<?phpheader("name:anleb");?>
客户端获取到了服务端设置的头信息[name]
那么这个有什么用呢？其实有判断对方到底是什么请求？是同步请求呢，还是异步请求
2.html页面是请求页面
<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"/></head><script>function CallAjax(){ var xmlHttp = new XMLHttpRequest(); xmlHttp.open("POST", '1.php', false); xmlHttp.setRequestHeader("Is_Ajax", "yes"); xmlHttp.send("name=anleb"); result = xmlHttp.responseText; alert("Result:" + result);}</script><body><input type='button' value='点击Ajax' onclick='CallAjax();'></body></html>返回页面
<?phpif(isset(getallheaders()['is_ajax']) && getallheaders()['is_ajax']=='yes'){ echo 'yes';}else{ echo 'no';}?>结果：
完毕，说的不好大家见谅。
