Accessing XML like an array (readonly)

FrozenXmlArrayAccess.php

<?php

class FrozenXmlArrayAccess implements ArrayAccess
{
    /**
     * @var SimpleXMLElement
     */
    private $xml;

    /**
     * @param SimpleXMLElement $xml
     */
    public function __construct(SimpleXMLElement $xml)
    {
        $this->xml = $xml;
    }

    /**
     * @param string|int $offset
     *
     * @return SimpleXMLElement
     */
    private function getElement($offset)
    {
        if (false !== $this->xml->$offset->asXML()) {
            $xml = $this->xml->$offset;
        }
        else {
            $xml = $this->xml[$offset];
        }

        return $xml;
    }

    /**
     * @param mixed $offset
     *
     * @return bool
     */
    public function offsetExists($offset)
    {
        return !is_null($this->getElement($offset));
    }

    /**
     * @param mixed $offset
     *
     * @return FrozenXmlArrayAccess
     *
     * @throws RuntimeException
     */
    public function offsetGet($offset)
    {
        if ($this->xml->getName() == $offset) {
            return $this;
        }

        $xml = $this->getElement($offset);

        if (is_null($xml)) {
            throw new RuntimeException('unknown node or attribute "' . $offset . '"');
        }

        return new self($xml);
    }

    /**
     * This method is not used since this class is readonly
     *
     * @param mixed $offset
     * @param mixed $value
     *
     * @throws RuntimeException
     */
    public function offsetSet($offset, $value)
    {
        throw new RuntimeException('This is readonly. writing to "' . (string) $offset . '" with value "' . (string) $value . '" not possible');
    }

    /**
     * This method is not used since this class is readonly
     *
     * @param mixed $offset
     *
     * @throws RuntimeException
     */
    public function offsetUnset($offset)
    {
        throw new RuntimeException('This is readonly. Unsetting "' . (string) $offset . '" not possible');
    }

    /**
     * @return string
     */
    public function __toString()
    {
        return (string) $this->xml;
    }
}