here you can get additional data like the transaction id from the transaction details,
If key not exists return null.

@param $key

@return mixed|null
430716
public function getAdditionalData($key)
    {
        if (true === isset($this->additionalData[$key])) {
            return $this->additionalData[$key];
        }

        return null;
    }
919159
public function get($key)
    {
        if (!$key || !isset($this->sessionData[$key])) {
            return null;
        }

        return $this->sessionData[$key];
    }
262361
public function getPayload($key = null)
    {
        if (is_null($key)) {
            return $this->payload;
        }

        return isset($this->payload[$key]) ? $this->payload[$key] : null;
    }
1050692
public function getValue($key) {
		if (isset ( $this->a [$key] )) {
			return $this->a [$key];
		} else {
			return NULL;
		}
	}
1021280
private function get($key) {
		if (isset($_SESSION[self::SESSION_KEY_FLASH]) && isset($_SESSION[self::SESSION_KEY_FLASH][$key])) {
			$message = $_SESSION[self::SESSION_KEY_FLASH][$key];

			unset($_SESSION[self::SESSION_KEY_FLASH][$key]);

			return $message;
		}
		else {
			return null;
		}
	}
547685
public function get($key){
		if(DEBUG_MODE){
			BDebug::message('[Memcached]: Get key('.$key.')');
			}
		$mc=$this->getMemcached();
		if(empty($mc))return NULL;
		$this->queries_get_count++;
		return $mc->get($key);
		}
92867
public function get($key = null)
    {
        if (Arr::key($key, $this->_data)) {
            return $this->_data[$key];
        }

        return $this->_data;
    }
850702
public function getData($key = null)
    {
        if (!$key) {
            return $this->data;
        } else if ($this->hasKey($key)) {
            return $this->data[$key] ?? null;
        }

        return null;
    }
261462
protected function get($key)
    {
        $this->checkForNonEmptyKey($key);
        if (!$this->settingsHaveBeenRetrieved) {
            $this->retrieveSettings();
        }
        if (!isset($this->cachedSettings[$key])) {
            return null;
        }

        return $this->cachedSettings[$key];
    }
316772
public function get($key)
    {
        $item = $this->pool->getItem($key);
        if ($item->isMiss() === false) {
            return $this->serializer->deserialize($item->get());
        } else {
            return null;
        }
    }
1041258
public function get_data($key) {
        if (isset($this->data->$key)) {
            return $this->data->$key;
        }
        return null;
    }
221173
public function get($key)
    {
        if (!$this->isEnabled()) {
            return null;
        }

        if (xcache_isset($this->prefix() . $key)) {
            return unserialize(xcache_get($this->prefix() . $key));
        } else {
            return null;
        }
    }
1013803
protected function retrieve($key)
        {
            $cache = $this->db()->select("entity_name = '" . md5($this->_namespace) . "' AND table_name = '" . md5($key) . "'", true);

            if (!empty($cache)) {
                $data = unserialize($cache->getData());
                $infos = explode('%%%%%:::::', $data);
                if (time() >= current($infos)) {
                    return $this->forget($key);
                }

                return end($infos);
            }
            return null;
        }
994571
public function get($key)
    {
        if ($this->willRemove($key)) {
            return null;
        }
        if (true === $this->willChange($key)) {
            return $this->getCurrent($key);
        }
        return $this->getOriginal($key);
    }
281408
public function get(string $key)
    {
        if (!isset($this->data[$key])) {
            return null;
        }

        $struct = $this->data[$key];

        if ($struct->ttl !== null) {
            $expire = \time() + $struct->ttl;
            if ($struct->expire < $expire) {
                $struct->expire = $expire;
                $this->queue->insert([$key, $struct->expire], -$struct->expire);
            }
        }

        return $struct->data;
    }
261532
public function get($key) {
        if (!isset($this->cache[$key]) OR $this->cache[$key]['ttl'] <= $this->getNow()) {
            return null;
        }
        return $this->cache[$key]['value'];
    }
179621
public function offsetGet($key) {
		$hashCode = HashUtils::hashCode($key);
		if (isset($this->values[$hashCode])) {
			return $this->values[$hashCode];
		}
		return null;
	}