What are some alternative methods or functions in PHP that can be used to calculate end dates based on specified durations?

When working with dates in PHP, it can be useful to calculate end dates based on specified durations. One common way to achieve this is by using the `strtotime()` function in combination with the `date()` function. By specifying a start date and adding a duration in seconds, we can easily calculate the end date. Another approach is to use the `DateTime` class, which provides a more object-oriented way to work with dates and times in PHP.

// Using strtotime() and date() function
$start_date = '2022-01-01';
$duration = 86400; // 1 day in seconds
$end_date = date('Y-m-d', strtotime($start_date) + $duration);
echo $end_date;

// Using DateTime class
$start_date = new DateTime('2022-01-01');
$duration = new DateInterval('P1D'); // 1 day
$start_date->add($duration);
$end_date = $start_date->format('Y-m-d');
echo $end_date;