src/PEAR/Dependency2.php
function validateOsDependency($dep)
    {
        if ($this->_state != PEAR_VALIDATE_INSTALLING && $this->_state != PEAR_VALIDATE_DOWNLOADING) {
            return true;
        }

        if ($dep['name'] == '*') {
            return true;
        }

        $not = isset($dep['conflicts']) ? true : false;
        switch (strtolower($dep['name'])) {
            case 'windows' :
                if ($not) {
                    if (strtolower(substr($this->getPHP_OS(), 0, 3)) == 'win') {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Cannot install %s on Windows");
                        }

                        return $this->warning("warning: Cannot install %s on Windows");
                    }
                } else {
                    if (strtolower(substr($this->getPHP_OS(), 0, 3)) != 'win') {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Can only install %s on Windows");
                        }

                        return $this->warning("warning: Can only install %s on Windows");
                    }
                }
            break;
            case 'unix' :
                $unices = array('linux', 'freebsd', 'darwin', 'sunos', 'irix', 'hpux', 'aix');
                if ($not) {
                    if (in_array($this->getSysname(), $unices)) {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Cannot install %s on any Unix system");
                        }

                        return $this->warning( "warning: Cannot install %s on any Unix system");
                    }
                } else {
                    if (!in_array($this->getSysname(), $unices)) {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Can only install %s on a Unix system");
                        }

                        return $this->warning("warning: Can only install %s on a Unix system");
                    }
                }
            break;
            default :
                if ($not) {
                    if (strtolower($dep['name']) == strtolower($this->getSysname())) {
                        if (!isset($this->_options['nodeps']) &&
                              !isset($this->_options['force'])) {
                            return $this->raiseError('Cannot install %s on ' . $dep['name'] .
                                ' operating system');
                        }

                        return $this->warning('warning: Cannot install %s on ' .
                            $dep['name'] . ' operating system');
                    }
                } else {
                    if (strtolower($dep['name']) != strtolower($this->getSysname())) {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError('Cannot install %s on ' .
                                $this->getSysname() .
                                ' operating system, can only install on ' . $dep['name']);
                        }

                        return $this->warning('warning: Cannot install %s on ' .
                            $this->getSysname() .
                            ' operating system, can only install on ' . $dep['name']);
                    }
                }
        }
        return true;
    }
function validateOsDependency($dep)
    {
        if ($this->_state != PEAR_VALIDATE_INSTALLING && $this->_state != PEAR_VALIDATE_DOWNLOADING) {
            return true;
        }

        if ($dep['name'] == '*') {
            return true;
        }

        $not = isset($dep['conflicts']) ? true : false;
        switch (strtolower($dep['name'])) {
            case 'windows' :
                if ($not) {
                    if (strtolower(substr($this->getPHP_OS(), 0, 3)) == 'win') {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Cannot install %s on Windows");
                        }

                        return $this->warning("warning: Cannot install %s on Windows");
                    }
                } else {
                    if (strtolower(substr($this->getPHP_OS(), 0, 3)) != 'win') {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Can only install %s on Windows");
                        }

                        return $this->warning("warning: Can only install %s on Windows");
                    }
                }
            break;
            case 'unix' :
                $unices = array('linux', 'freebsd', 'darwin', 'sunos', 'irix', 'hpux', 'aix');
                if ($not) {
                    if (in_array($this->getSysname(), $unices)) {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Cannot install %s on any Unix system");
                        }

                        return $this->warning( "warning: Cannot install %s on any Unix system");
                    }
                } else {
                    if (!in_array($this->getSysname(), $unices)) {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError("Can only install %s on a Unix system");
                        }

                        return $this->warning("warning: Can only install %s on a Unix system");
                    }
                }
            break;
            default :
                if ($not) {
                    if (strtolower($dep['name']) == strtolower($this->getSysname())) {
                        if (!isset($this->_options['nodeps']) &&
                              !isset($this->_options['force'])) {
                            return $this->raiseError('Cannot install %s on ' . $dep['name'] .
                                ' operating system');
                        }

                        return $this->warning('warning: Cannot install %s on ' .
                            $dep['name'] . ' operating system');
                    }
                } else {
                    if (strtolower($dep['name']) != strtolower($this->getSysname())) {
                        if (!isset($this->_options['nodeps']) && !isset($this->_options['force'])) {
                            return $this->raiseError('Cannot install %s on ' .
                                $this->getSysname() .
                                ' operating system, can only install on ' . $dep['name']);
                        }

                        return $this->warning('warning: Cannot install %s on ' .
                            $this->getSysname() .
                            ' operating system, can only install on ' . $dep['name']);
                    }
                }
        }
        return true;
    }
Specify a dependency on an OS.  Use arch for detailed os/processor information

There are two generic OS dependencies that will be the most common, unix and windows.
Other options are linux, freebsd, darwin (OS X), sunos, irix, hpux, aix