src/Primitive/Container/Container.php
protected function getKey($key, $default = null)
    {
        return ($this->isIntegerable($key, true)) ? $this->getIntegerable($key, $default)
                                                  : $this->getStringable($key, $default);
    }
protected function getKey($key, $default = null)
    {
        return ($this->isIntegerable($key, true)) ? $this->getIntegerable($key, $default)
                                                  : $this->getStringable($key, $default);
    }
Retrieve proper key.

@param mixed $key
@param null|mixed $default
@return int|string