$start = $hours->start{$day};

//For example
$start = $hours->startSunday;

// will echo something like 8:00am
echo $hours->startSunday;

//will echo something like 7:00am
echo $hours->startMonday;

//so something like
echo $hours->start.$day;

$hours = new stdClass();
$day = 'Sunday';
$hours->{'start'.$day} = 10;
$start = $hours->{'start'.$day};
var_dump($start);

class Hours
    {
    private $hours = array();
    public function getStart($day)
        {
        return $this->hours[$day];
        }
    public function setStart($day, $value)
        {
        $this->hours[$day] = $value;
        }
    }
$hours = new Hours();
$day = 'Sunday';
$hours->setStart($day, 10);
$start = $hours->getStart($day);
var_dump($start);

class Hours {
    private $data = array();

    public function __get($day) {
        return $this->data[$day];
    }

    public function __set($day, $val) {
        $this->data[$day] = $val;
    }
}

$h = new Hours();
echo $hours->startMonday;