code: public static function loadClass(string $className)
      {
          if (self::$isInit === false) {
              throw new BadUse('bootstrap doesn\'t seem to have been initialized');
          }
          $fileName = null;
          if (($namespacePos = strripos($className, '\\')) !== false) {
              $namespacePos = (int) $namespacePos; // re: https://github.com/phpstan/phpstan/issues/647
              $namespace = substr($className, 0, $namespacePos);
              $subNamespaces = array();
              while ($fileName === null && $namespace != null) {
                  if (isset(self::$namespaces[$namespace]) === false) {
                      $subNamespacePos = strripos($namespace, '\\');
                      $subNamespacePos = (int) $subNamespacePos; // re: https://github.com/phpstan/phpstan/issues/647
                      $subNamespaces[] = substr($namespace, $subNamespacePos);
                      $namespace = substr($namespace, 0, $subNamespacePos);
                  } else {
                      $fileName = self::$namespaces[$namespace];
                  }
              }
              if ($fileName === null) {
                  throw new UnknownNamespace('can\'t find namespace "' . substr($className, 0, $namespacePos) . '"');
              }
              $fileName = self::$namespaces[$namespace] . str_replace('\\', DIRECTORY_SEPARATOR, implode('', array_reverse($subNamespaces)));
              $className = substr($className, $namespacePos + 1);
          }
          if ($fileName != null) {
              $fileName .= DIRECTORY_SEPARATOR . str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';
              require $fileName;
          }
      }
docs: Autoloader registered function.
  Try to require a file according to the needed class.
  
  @static
  @throws BadUse           if bootstrap isn't initialized
  @throws UnknownNamespace if needed namespace can't be found
  @param  string           $className needed class name