$_SERVER is superglobal array that holds the following information
1. $_SERVER[‘PHP_SELF’]—> it shows the filename of the currently executing script.
2.$_SERVER[‘GATEWAY_INTERFACE’]—> what revision of the CGI Specification of the server for exampke ‘CGI1.1’
3.$_SERVER[‘SERVER_ADDR’]–> IP address of the server underwhich the current script is executing
4.$_SERVER[‘SERVER_NAME’]–> The name of the server host under which the current script is executing.
5.$_SERVER[‘SERVER_SOFTWARE’]–>server identification string for example APche1.0
6.$_SERVER[‘SERVER_PROTOCOL]–> Name and revision of the information protocol via which the page was reqested For example ‘HTTP/1.0’
7.$_SERVER[‘REQUEST_METHOD’]–> it shows which request method was used to access the page Eg: GET,HEAD,POST,PUT
8.$_SERVER[‘REMOTE_ADDR’]–> IP address of the user is viewing the current page.
For more varibles details refer http://php.net