src/Primitive/Container/Container.php
public function mergeWithKey($items, $key, $default = null)
    {
        $key = $this->getKey($key);

        if ( ! $this->has($key)) throw new InvalidArgumentException('Key: '.$key.' not exists');

        $get = $this->get($key, $default);

        $value = array_merge($get, $this->retrieveValue($items));

        return $this->copy()->set($key, $value);
    }
public function mergeWithKey($items, $key, $default = null)
    {
        $key = $this->getKey($key);

        if ( ! $this->has($key)) throw new InvalidArgumentException('Key: '.$key.' not exists');

        $get = $this->get($key, $default);

        $value = array_merge($get, $this->retrieveValue($items));

        return $this->copy()->set($key, $value);
    }
Merge Arrayable with specified key in Container.

@param mixed $items
@param mixed $key
@param null $default
@return $this
@throws InvalidArgumentException