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My request $_SERVER vars
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$_SERVER variables,proxy,request headers,all request variables

You can find in this page something like php print_r($_SERVER) or phpinfo (it's in PHP language), so it's your request headers and server headers. (information from $_SERVER,$_SERVER info, my request headers)
$_SERVER is an array with not fixed size, number of variables linked with request type and servers settings. This array fully generated by web server application, that is why it is different for Apache and Nginx server. Varibales of PHP $_SERVER array can be replaced during php and web server execution, nginx can rewite some variables from external Apache server (for example). fast link:
The most useful variables that sending user's browser:
HTTP_ACCEPT_CHARSET - Contents of the Accept-Charset: header from the current request, if there is one. Example: 'iso-8859-3,*,utf-8'.
HTTP_ACCEPT_ENCODING - Contents of the Accept-Encoding: header from the current request, if there is one. Example: 'jpeg'.
HTTP_ACCEPT_LANGUAGE - Contents of the Accept-Language: header from the current request, if there is one. Example: 'en'.
HTTP_REFERER - The address of the page (if any) which referred the user agent to the current page. This is set by the user agent. Not all user agents will set this, and some provide the ability to modify HTTP_REFERER as a feature. In short, it cannot really be trusted.
HTTP_USER_AGENT - Contents of the User-Agent: header from the current request, if there is one. This is a string denoting the user agent being which is accessing the page.
REMOTE_ADDR - The IP address from which the user is viewing the current page. (can be modified by proxy)
$_SERVER is an array containing information such as headers, paths, and script locations (request information + part of server execution information). The most useful header it's a REMOTE_ADDR, it containt IP address of remove computer (or remote proxy) that using in 95% of web sites. There are two simple ways to make user's IP address invisible: proxy and VPN.
 WIKI links : what is my MAC address, how do i find the MAC address of my computer
determine mac address from ip, mac address lookup, find ping a remote mac address
Your request headers

USERmk
HOME/home/mk
FCGI_ROLERESPONDER
SCRIPT_NAME/my_server.php
PATH_INFO/my_server.php
QUERY_STRING
CONTENT_TYPE
CONTENT_LENGTH
SERVER_ADDR91.197.129.127
SERVER_PORT80
SERVER_PROTOCOLHTTP/1.0
GATEWAY_INTERFACECGI/1.1
REQUEST_URI/all-request-varibles-and-my-request-data/php-request-data/my_server.php
REQUEST_METHODGET
REMOTE_USER
REMOTE_ADDR54.197.171.35
REMOTE_PORT60150
HTTP_HOSTmy-addr.com
HTTP_ACCEPT_ENCODINGx-gzip, gzip, deflate
HTTP_USER_AGENTCCBot/2.0 (http://commoncrawl.org/faq/)
HTTP_ACCEPTtext/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
PHP_SELF/my_server.php/my_server.php
REQUEST_TIME_FLOAT1481046441.8789
REQUEST_TIME1481046441