gistfile1.txt

class OutputWriter
{
    private $_closure;

    public function __construct($closure = null)
    {
        if ($closure === null) {
            $closure = function($message) {};
        }
        $this->_closure = $closure;
    }

    public function write($message)
    {
        $closure = $this->_closure;
        $closure($message);
    }
}