There are some names you can not use here:

- APP_NAME
- DOCUMENT_ROOT
- FCGI_ROLE
- GATEWAY_INTERFACE
- GROUP
- HOME
- PATH
- PS1
- QUERY_STRING
- SESSION
- USER

Also names cannot begin with:

- HTTP_
- PHP_
- REDIRECT_
- REMOTE_
- REQUEST_
- SCRIPT_
- SERVER_
- LC_