$_SERVER

$_SERVER is an array containing information such as headers, paths, and script locations. The entries in this array are created by the web server. There is no guarantee that every web server will provide any of these; servers may omit some, or provide others not listed here. That said, a large number of these variables are accounted for in the » CGI/1.1 specification, so you should be able to expect those.

$HTTP_SERVER_VARS contains the same initial information, but is not a superglobal. (Note that $HTTP_SERVER_VARS and $_SERVER are different variables and that PHP handles them as such)

PHP_SELF

The filename of the currently executing script, relative to the document root. For instance, $_SERVER['PHP_SELF'] in a script at the address http://example.com/test.php/foo.bar would be /test.php/foo.bar. The __FILE__ constant contains the full path and filename of the current (i.e. included) file. If PHP is running as a command-line processor this variable contains the script name since PHP 4.3.0. Previously it was not available.
$_SERVER['PHP_SELF'];

GATEWAY_INTERFACE
What revision of the CGI specification the server is using; i.e. 'CGI/1.1'.
$_SERVER['GATEWAY_INTERFACE'];

SERVER_ADDR
The IP address of the server under which the current script is executing.
$_SERVER['SERVER_ADDR'];

SERVER_NAME
The name of the server host under which the current script is executing. If the script is running on a virtual host, this will be the value defined for that virtual host.
$_SERVER['SERVER_NAME'];

SERVER_PROTOCOL
Name and revision of the information protocol via which the page was requested; i.e. 'HTTP/1.0';
$_SERVER['SERVER_PROTOCOL'];

REQUEST_METHOD
Which request method was used to access the page; i.e. 'GET', 'HEAD', 'POST', 'PUT'
Note:

PHP script is terminated after sending headers (it means after producing any output without output buffering) if the request method was HEAD.
$_SERVER['REQUEST_METHOD'];

REQUEST_TIME
The timestamp of the start of the request. Available since PHP 5.1.0. It is float with microseconds since PHP 5.4.0.
$_SERVER['REQUEST_TIME'];

QUERY_STRING
The query string, if any, via which the page was accessed.
$_SERVER['QUERY_STRING'];

DOCUMENT_ROOT
The document root directory under which the current script is executing, as defined in the server's configuration file.
$_SERVER['DOCUMENT_ROOT'];
eg: c:\xampp\htdocs

HTTP_ACCEPT
Contents of the Accept: header from the current request, if there is one.
$_SERVER['HTTP_ACCEPT'];

HTTP_ACCEPT_CHARSET
Contents of the Accept-Charset: header from the current request, if there is one. Example: 'iso-8859-1,*,utf-8'.
$_SERVER['HTTP_ACCEPT_CHARSET'];

HTTP_ACCEPT_ENCODING
Contents of the Accept-Encoding: header from the current request, if there is one. Example: 'gzip'.
$_SERVER['HTTP_ACCEPT_ENCODING'];

HTTP_ACCEPT_LANGUAGE
Contents of the Accept-Language: header from the current request, if there is one. Example: 'en'.
$_SERVER['HTTP_ACCEPT_LANGUAGE'];

HTTP_CONNECTION
Contents of the Connection: header from the current request, if there is one. Example: 'Keep-Alive'.
$_SERVER['HTTP_CONNECTION'];

HTTP_HOST
Contents of the Host: header from the current request, if there is one.
$_SERVER['HTTP_HOST'];

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.
$_SERVER['HTTP_REFERER'];

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. A typical example is: Mozilla/4.5 [en] (X11; U; Linux 2.2.9 i586). Among other things, you can use this value with get_browser() to tailor your page's output to the capabilities of the user agent.
$_SERVER['HTTP_USER_AGENT'];

HTTPS
Set to a non-empty value if the script was queried through the HTTPS protoco

REMOTE_ADDR
The IP address from which the user is viewing the current page.
$_SERVER['REMOTE_ADDR'];
eg:127.0.0.1

REMOTE_HOST
The Host name from which the user is viewing the current page. The reverse dns lookup is based off the REMOTE_ADDR of the user.
$_SERVER['REMOTE_HOST'];

REMOTE_PORT
The port being used on the user's machine to communicate with the web server.
$_SERVER['REMOTE_PORT'];

SCRIPT_NAME
Contains the current script's path. This is useful for pages which need to point to themselves. The __FILE__ constant contains the full path and filename of the current (i.e. included) file.
$_SERVER['SCRIPT_NAME'];

REQUEST_URI
The URI which was given in order to access this page; for instance, '/index.html'.