DefaultOptions.php

<?php

class DefaultOptions extends Options
{
    setRequired()
    setDefined()
    resolve()
}

Options.php

<?php

class Options implements ArrayAccess, Countable
{
    get()
    set()
    has()
    merge()
    replace()
}

usage.php

<?php

class Mailer
{
    // ...
    public function __construct(array $options = array())
    {
        $defaultOptions = new DefaultOptions();
        $this->setDefaultOptions($defaultOptions);

        $this->options = $defaultOptions->resolve($options);
    }

    protected function setDefaultOptions(DefaultOptions $options)
    {
        $options['host'] = 'smtp.example.org';
        $options['username'] = 'user';
        $options['password'] = 'pa$$word';
        $options['port'] = function (Options $options) {
            // ...
        };
        $options['encryption'] = null;
    }
}