Variables in PHP are represented by a dollar sign followed by the variable name. The variable name is case-sensitive.

Variable names follow the same rules as other labels in PHP. The name must start with a letter or underscore, followed by any number of letters, numbers, or underscores. Valid characters for a variable name are '[a-zA-Z_x7f-xff][a-zA-Z0-9_x7f-xff]*' (sans quotes).

There is a new set of powerful predefined arrays that contain variables from the webserver, user-input and the environment. These arrays are known as superglobals. Here is a listing of superglobals:

$GLOBALS	Contains a reference to every variable that is available within the global scope of the script.
$_SERVER	Variables set by the web server and/or otherwise directly related to the execution environment of the script.
$_GET	Variables provided to the script via a URL query string.
$_POST	Variables provided to the script via the HTTP POST.
$_COOKIE	Variables provided to the script via HTTP COOKIES.
$_FILES	Variables provided to the script via HTTP POST file uploads.
$_ENV	Variables provided to the script via the environment
$_REQUEST	Variables provided to the script via the GET, POST, and COOKIE inputs. These variables cannot be trusted.
$_SESSION	Variables which are currently registered to a scripts's session