public function setCallback($env, callable $callable = null)
    {
        $this->swapCallable($env, $callable, $this->running_env);
        
        $this->callbacks[$env] = $callable;
        if (empty($this->errors[$env])) {
            $this->setCoreErrors($env, self::getDefaultCoreErrors());
        }
        if (empty($this->exceptions[$env])) {
            $this->setUncaughtExceptions($env, self::$default_exceptions);
        }
        
        return $this;
    }