src/Transaction.php
public function offsetGet($offset)
    {
        $txKey = isset($this->attributeMap[$offset]) ? $this->attributeMap[$offset] : null;

        if (is_array($txKey) && isset($this->txData[$txKey['key']])) {
            return $this->txData[$txKey['key']];
        }
        return null;
    }
public function offsetGet($offset)
    {
        $txKey = isset($this->attributeMap[$offset]) ? $this->attributeMap[$offset] : null;

        if (is_array($txKey) && isset($this->txData[$txKey['key']])) {
            return $this->txData[$txKey['key']];
        }
        return null;
    }
offsetGet

@param string $offset
@return mixed