有没有办法逻辑比较PHP DateTime对象？

Question

This is part of a simple project that logs payroll hours as a datetime object in MySQL 5.5. I am trying to compare two datetime values to see if they are at least 30 minutes apart. Sounds simple enough but the value of $lastshiftend keeps being set to the same value as $mealendtime and I don't see where or how. That seems to be the only problem but there definitely could be other things I am missing. TIA.

if ($result = mysqli_query($conn, $select)) {
$row_count = mysqli_num_rows($result);
if($row_count == 1) {
    // 30 Minute Meal Break
    $lastshiftend = new DateTime($row[3]);
    $mealendtime = new DateTime('NOW');
    $mealtime = $mealendtime->diff($lastshiftend);
    $mealminend = $lastshiftend->add(new DateInterval(PT30M));
    // $mealminend = $lastshiftend->modify('+ 30 minute');

    if($mealendtime < $mealminend) {
        echo '<br>Colorado State law requires meal breaks to be at least 30 minutes in length.';
        echo '<hr><a href="./index.html">Main Menu</a>';
    } else {
        echo 'Log it!';
        // header("Location: ./ActionClockIn.php");
    }       
} else {
    echo 'Error! If you ended up here something broke!.';
    echo '<hr><a href="./index.html">Main Menu</a>';
}

}

Answer 1

Unless you use DateTimeImmutable() your DateTime objects will be modified when you call methods like DateTime::add() or DateTime::modify()

$lastshiftend = new DateTimeImmutable($row[3]);
// Now $lastshiftend is unchanged
$mealminend = $lastshiftend->add(new DateInterval(PT30M));

It looks like you will need that for both DateTime objects

$lastshiftend = new DateTimeImmutable($row[3]);
$mealendtime = new DateTimeImmutable(); //"NOW" is not necessary
$mealtime = $mealendtime->diff($lastshiftend);
$mealminend = $lastshiftend->add(new DateInterval(PT30M));