PHP provides a large number of predefined variables to any script which it runs. Many of these variables, however, cannot be fully documented as they are dependent upon which server is running, the version and setup of the server, and other factors. Some of these variables will not be available when PHP is run on the command-line.
Despite these factors, here is a list of predefined variables available under a stock installation of PHP 3 running as a module under a stock installation of Apache 1.3.6.
For a list of all predefined variables (and lots of other useful information), please see (and use) phpinfo().
Note: This list is neither exhaustive nor intended to be. It is simply a guideline as to what sorts of predefined variables you can expect to have access to in your script.
These variables are created by the Apache webserver. If you are running another webserver, there is no guarantee that it will provide the same variables; it may omit some, or provide others not listed here. That said, a large number of these variables are accounted for in the CGI 1.1 specification, so you should be able to expect those.
Note that few, if any, of these will be available (or indeed have any meaning) if running PHP on the command line.
What revision of the CGI specification the server is using; i.e. 'CGI/1.1'.
The name of the server host under which the current script is executing. If the script is running on a virtual host, this will be the value defined for that virtual host.
Server identification string, given in the headers when responding to requests.
Name and revision of the information protocol via which the page was requested; i.e. 'HTTP/1.0';
Which request method was used to access the page; i.e. 'GET', 'HEAD', 'POST', 'PUT'.
The query string, if any, via which the page was accessed.