private function getConstants(): void
  {
    // If myTargetConfigFilename is not set return immediately.
    if (!isset($this->constantClassName)) return;

    $reflection = new \ReflectionClass($this->constantClassName);

    $constants = $reflection->getConstants();
    foreach ($constants as $name => $value)
    {
      if (!is_numeric($value)) $value = "'".$value."'";

      $this->replacePairs['@'.$name.'@'] = $value;
    }

    $this->io->text(sprintf('Read %d constants for substitution from <fso>%s</fso>',
                            sizeof($constants),
                            OutputFormatter::escape($reflection->getFileName())));
  }