The following is a brief overview of the standard environment variables you're likely to encounter. Each server implements the majority of them consistently, but variations, exceptions, and additions exist. In general, you're more likely to find a new, otherwise undocumented variable omitted rather than a documented variable. The only way to be sure, however, is to check your server's documentation.
This section is taken from the NCSA specifications (see Figure 28.1) and is the closest thing to "standard" as you'll find. The following environment variables are set each time the server launches an instance of your script and are private and specific to that instance: