以下は最適化されたコードである：

```

function countWeekendDaysOptimized($startDate, $endDate) {
    // Convert start and end dates to Unix timestamps
    $start = strtotime($startDate);
    $end = strtotime($endDate);
    // Calculate the number of days between the two dates
    $days = ceil(abs($end - $start) / 86400);
    // Calculate the number of weekend days between the two dates
    $weekendDays = ceil(($days + date('w', $start) - 1) / 7) * 2;
    // If the start date is a Sunday, subtract one from the total number of weekend days
    if (date('w', $start) == 0) {
        $weekendDays--;
    }
    // If the end date is a Saturday, subtract one from the total number of weekend days
    if (date('w', $end) == 6) {
        $weekendDays--;
    }
    // Return the total number of weekend days
    return $weekendDays;
}


```
Here is an optimized code:

```

function countWeekendDaysOptimized($startDate, $endDate) {
    // Convert start and end dates to Unix timestamps
    $start = strtotime($startDate);
    $end = strtotime($endDate);
    // Calculate the number of days between the two dates
    $days = ceil(abs($end - $start) / 86400);
    // Calculate the number of weekend days between the two dates
    $weekendDays = ceil(($days + date('w', $start) - 1) / 7) * 2;
    // If the start date is a Sunday, subtract one from the total number of weekend days
    if (date('w', $start) == 0) {
        $weekendDays--;
    }
    // If the end date is a Saturday, subtract one from the total number of weekend days
    if (date('w', $end) == 6) {
        $weekendDays--;
    }
    // Return the total number of weekend days
    return $weekendDays;
}


```