Symfony\Component\DependencyInjection\ContainerBuilder::getAlias PHP Method

    public function getAlias($id)
    {
        $id = strtolower($id);

        if (!isset($this->aliasDefinitions[$id])) {
            throw new InvalidArgumentException(sprintf('The service alias "%s" does not exist.', $id));
        }

        return $this->aliasDefinitions[$id];
    }

 /**
  * @param string $id
  * @return string
  */
 public function definitionClass(string $id) : string
 {
     if ($this->builder->hasAlias($id)) {
         return $this->builder->getDefinition((string) $this->builder->getAlias($id))->getClass()->getClass();
     }
     return $this->builder->getDefinition($id)->getClass();
 }