public function has($id, $checkInstance = false)
    {
        return $checkInstance ? isset($this->_components[$id]) : isset($this->_definitions[$id]);
    }
public function has($id, $checkInstance = false)
    {
        return $checkInstance ? isset($this->_components[$id]) : isset($this->_definitions[$id]);
    }
Returns a value indicating whether the locator has the specified component definition or has instantiated the component.
This method may return different results depending on the value of `$checkInstance`.

- If `$checkInstance` is false (default), the method will return a value indicating whether the locator has the specified
component definition.
- If `$checkInstance` is true, the method will return a value indicating whether the locator has
instantiated the specified component.

@param string $id component ID (e.g. `db`).
@param bool $checkInstance whether the method should check if the component is shared and instantiated.
@return bool whether the locator has the specified component definition or has instantiated the component.
@see set()