What is $_SERVER ??

$_SERVER is a superglobal array in PHP. It contains following values:

    $_SERVER['UNIQUE_ID']
    $_SERVER['HTTP_HOST']
    $_SERVER['HTTP_USER_AGENT']
    $_SERVER['HTTP_ACCEPT']
    $_SERVER['HTTP_ACCEPT_LANGUAGE']
    $_SERVER['HTTP_ACCEPT_ENCODING']
    $_SERVER['HTTP_ACCEPT_CHARSET']
    $_SERVER['HTTP_KEEP_ALIVE']
    $_SERVER['HTTP_CONNECTION']
    $_SERVER['HTTP_CACHE_CONTROL']
    $_SERVER['PATH']
    $_SERVER['SERVER_SIGNATURE']
    $_SERVER['SERVER_SOFTWARE']
    $_SERVER['SERVER_NAME']
    $_SERVER['SERVER_ADDR']
    $_SERVER['SERVER_PORT']
    $_SERVER['REMOTE_ADDR']
    $_SERVER['DOCUMENT_ROOT']
    $_SERVER['SERVER_ADMIN']
    $_SERVER['SCRIPT_FILENAME']
    $_SERVER['REMOTE_PORT']
    $_SERVER['GATEWAY_INTERFACE']
    $_SERVER['SERVER_PROTOCOL']
    $_SERVER['REQUEST_METHOD']
    $_SERVER['QUERY_STRING']
    $_SERVER['REQUEST_URI']
    $_SERVER['SCRIPT_NAME']
    $_SERVER['PHP_SELF']
    $_SERVER['REQUEST_TIME']