Not documented here is the fact that $_SERVER is populated with some pretty useful information when accessing PHP via the shell.
["_SERVER"]=>
array(24) {
["MANPATH"]=>
string(48) "/usr/share/man:/usr/local/share/man:/usr/X11/man"
["TERM"]=>
string(11) "xterm-color"
["SHELL"]=>
string(9) "/bin/bash"
["SSH_CLIENT"]=>
string(20) "127.0.0.1 41242 22"
["OLDPWD"]=>
string(60) "/Library/WebServer/Domains/www.example.com/private"
["SSH_TTY"]=>
string(12) "/dev/ttys000"
["USER"]=>
string(5) "username"
["MAIL"]=>
string(15) "/var/mail/username"
["PATH"]=>
string(57) "/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin"
["PWD"]=>
string(56) "/Library/WebServer/Domains/www.example.com/www"
["SHLVL"]=>
string(1) "1"
["HOME"]=>
string(12) "/Users/username"
["LOGNAME"]=>
string(5) "username"
["SSH_CONNECTION"]=>
string(31) "127.0.0.1 41242 10.0.0.1 22"
["_"]=>
string(12) "/usr/bin/php"
["__CF_USER_TEXT_ENCODING"]=>
string(9) "0x1F5:0:0"
["PHP_SELF"]=>
string(10) "Shell.php"
["SCRIPT_NAME"]=>
string(10) "Shell.php"
["SCRIPT_FILENAME"]=>
string(10) "Shell.php"
["PATH_TRANSLATED"]=>
string(10) "Shell.php"
["DOCUMENT_ROOT"]=>
string(0) ""
["REQUEST_TIME"]=>
int(1247162183)
["argv"]=>
array(1) {
[0]=>
string(10) "Shell.php"
}
["argc"]=>
int(1)
}