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)
  }