There are large number of predefined variables available in PHP.
Basically predefined variables are those variables that are already defined in PHP and have some special meanings, you can not use any predefined variables as your custom variable.
The following table lists the PHP superglobal variables:
|$GLOBALS||This variable contains a reference to every variable, which is currently available within the global scope of script.|
|$_GET||This variable is an associative array of the variables passed to the current script via the HTTP GET method|
|$_SERVER||This variable is an array containing the information such as headers, paths, script locations, etc.|
|$_FILES||This variable is also an associative array of items uploaded to the current script via the HTTP POST method|
|$_POST||This variable is also an associative array of variables passed to the current script via the HTTP POST method|
|$_COOKIE||This is an associative array of variables passed to the current script via the HTTP cookies|
|$_REQUEST||This is an associative array consisting of the contents of $_GET, $_POST, and $_COOKIE|
|$_PHP_SELF||This is a string containing the PHP script file name in which it is called|
|$php_errormsg||This is a variable containing the text of the last error message that generated by the PHP|
|$_SESSION||This is an associative array containing the session variables available to the current script|