<form method="post">
        <label>Başlangıç Tarihi (YYYY-MM-DD):</label>
        <input type="date" name="startDate" required><br><br>
        
        <label>Bitiş Tarihi (YYYY-MM-DD):</label>
        <input type="date" name="endingDate" required><br><br>
        
        <input type="submit" value="Hesapla">
    </form>

    <?php
    if ($_SERVER["REQUEST_METHOD"] == "POST") {
        $startDate = $_POST['startDate'];
        $endingDate = $_POST['endingDate'];
        function dateCalc($startDate, $endingDate) {
            $date1 = new DateTime($startDate);
            $date2 = new DateTime($endingDate);
            $interval = $date1->diff($date2);
            return $interval->format('%a');
        }
        $result = dateCalc($startDate, $endingDate);
        echo "<h2>Toplam Gün Sayısı: $result</h2>";
    }
    ?>