$_SERVER 是 PHP 预定义变量之一，可以直接使用，它是一个包含了诸如头信息（header）、路径（path）及脚本位置（script locations）信息的数组。

在浏览器打印出 $_SERVER 数组的代码如下：

<?phpprint_r($_SERVER);?>

浏览器的输出结果如下：

Array
(
    [HTTP_HOST] => localhost
    [HTTP_CONNECTION] => keep-alive
    [HTTP_CACHE_CONTROL] => max-age=0
    [HTTP_ACCEPT] => text/html,application/xhtml+xml,application/xml;q=0.9, image/webp,*/*;q=0.8
    [HTTP_UPGRADE_INSECURE_REQUESTS] => 1
    [HTTP_USER_AGENT] => Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.94 Safari/537.36
    [HTTP_ACCEPT_ENCODING] => gzip, deflate, sdch
    [HTTP_ACCEPT_LANGUAGE] => zh-CN,zh;q=0.8
    [HTTP_COOKIE] => PHPSESSID=e1bbc84e23bf85691e7c5a4ab07ee0de; pgv_pvi=4369311744; pgv_si=s1775918080; CNZZDATA155540=cnzz_eid%3D1811041545- 1463297631-%26ntime%3D1463303031
    [PATH] => /usr/bin:/bin:/usr/sbin:/sbin
    [SERVER_SIGNATURE] =>
    [SERVER_SOFTWARE] => Apache/2.4.16 (Unix) PHP/7.0.5
    [SERVER_NAME] => localhost
    [SERVER_ADDR] => ::1
    [SERVER_PORT] => 80
    [REMOTE_ADDR] => ::1
    [DOCUMENT_ROOT] => /Library/WebServer/Documents
    [REQUEST_SCHEME] => http
    [CONTEXT_PREFIX] =>
    [CONTEXT_DOCUMENT_ROOT] => /Library/WebServer/Documents
    [SERVER_ADMIN] => you@example.com
    [SCRIPT_FILENAME] => /Library/WebServer/Documents/book/str.php
    [REMOTE_PORT] => 59377
    [GATEWAY_INTERFACE] => CGI/1.1
    [SERVER_PROTOCOL] => HTTP/1.1
    [REQUEST_METHOD] => GET
    [QUERY_STRING] =>
    [REQUEST_URI] => /book/str.php
    [SCRIPT_NAME] => /book/str.php
    [PHP_SELF] => /book/str.php
    [REQUEST_TIME_FLOAT] => 1463828978.149
    [REQUEST_TIME] => 1463828978
    [argv] => Array
    (
    )
    [argc] => 0
)