Or maybe, go further with some dynamic programming ?

```php
$serializer = strtolower($this->settings['serialize']);
if (!isset($this->_serializers[$serializer])) {
    // Exception
}
if ($serializer !== 'php' && !constant('Memcached::HAVE_'. strtoupper($serializer))) {
    // Exception
}

$this->_Memcached->setOption(Memcached::OPT_SERIALIZER, $serializer);
```
Saving one if condition