The manual page "Variables from outside PHP" (language.variables.external.php), under "HTML Forms (GET and POST)", doesn't mention the new $_* arrays, nor that track_vars is now always on.  I suggest revision to read something like the following:

"When a form is submitted to a PHP script, any variables from that form will be automatically made available to the script by PHP. If the track_vars configuration option is turned on (always, as of version 4.0.3), then these variables will be located in the associative arrays $HTTP_POST_VARS, $HTTP_GET_VARS, and/or $HTTP_POST_FILES, according to the source of the variable in question.  From version 4.1.0 onward, they will also be in the new superglobal arrays $_POST, $_GET and $_FILES; POST and GET values will also be in the composite array $_REQUEST.

"For more information on these variables, please read Predefined variables. 

[Example 7-1 snipped]

"When the above form is submitted, the value from the text input will be available in $HTTP_POST_VARS['username'] and, from version 4.1.0, $_POST['username'] and $_REQUEST['username']. If the register_globals configuration directive is turned on, then the variable will also be available as $username in the global scope."