PHP solution for fixed strings length

FixedString.php

<?php

/**
 * @author Wallace de Souza Vizerra <wallacemaxters@gmail.com>
 * @version 1.0
 * @see hehehehehe
 **/

final class FixedString implements ArrayAccess
{

    /**
     * @var string
     **/

    protected $text;

    /**
     * @var string
     * The encoding of string
     * */
    protected $encoding;

    /**
     * @var int
     **/
    protected $length;

    /**
     * @param string $text
     * @param int $length
     * @param null|string $encoding 
     **/
    public function __construct($text, $length, $encoding = null)
    {

        $this->encoding = $encoding ?: mb_internal_encoding();

        if (is_object($text) && method_exists($text, '__toString')) {

            $text = (string) $text;
            
        } elseif ( ! is_string($text)) {

            $message = sprintf('The first argument should be a string, %s given', gettype($text));

            throw new InvalidArgumentException($message);
        }

        $this->length = $length;

        $this->text = str_pad(mb_substr((string) $text, 0, $this->length, $this->encoding), $this->length);
    }

    /**
     * @return int
     **/
    public function getLength()
    {
        return $this->length;
    }

    /**
     * Returns the string lenght accordding to encoding
     * @return int
     **/
    public function getEncodedLength()
    {
        return mb_strlen($this->text, $this->encoding);
    }

    /**
     * Get encoding of string
     * @return string
     **/
    public function getEncoding()
    {
        return $this->encoding;
    }

    /**
     * Convert this object in to string
     * */
    public function __toString()
    {
        return $this->text;
    }

    /**
     * Implementation of \ArrayAccess interface
     * @return string
     * */
    public function offsetGet($index)
    {
        return mb_substr($this->text, $index, 1, $this->encoding);
    }

    /**
     * @param int $index
     * @param string $value
     * @return void
     * */
    public function offsetSet($index, $value)
    {
        if ($index > $this->getEncodedLength()) {

            throw new OutOfRangeException('Out of Range!');
        }

        // Não existe mb_substr_replace :\
        // mb_substr_replace doesn't exists :\

        $text = mb_substr($this->text, 0, $index, $this->encoding) 
                      . $value
                      . mb_substr($this->text, $index + 1, $this->getLength(), $this->encoding);

        $this->text = $text;
    }

    /**
     * @param int $index
     * @return void
     * */
    public function offsetUnset($index)
    {
        $this->offsetSet($index, ' ');
    }

    /**
     * @param int $index
     * @return boolean
     **/
    public function offsetExists($index)
    {
        return $this->offsetGet($index) != '';
    }
}