php function returned but executing

function returned but execution is running, i tried use else but it still so

    public function getClass() : string
    {
        $class = require_once $this->plugin;
        // i tried this here is returning true: if (class_exists($class)) exit($class);
        if (class_exists($class)) return $class;
        throw new Exception\PluginClassNotFoundException(); // but here is executing??
        // i tried using else
    }