whats the difference between using date function and datetime in php

1//get Date diff as intervals 
2$d1 = new DateTime("2018-01-10 00:00:00");
3$d2 = new DateTime("2019-05-18 01:23:45");
4$interval = $d1->diff($d2);
5$diffInSeconds = $interval->s; //45
6$diffInMinutes = $interval->i; //23
7$diffInHours   = $interval->h; //8
8$diffInDays    = $interval->d; //21
9$diffInMonths  = $interval->m; //4
10$diffInYears   = $interval->y; //1
11
12//or get Date difference as total difference
13$d1 = strtotime("2018-01-10 00:00:00");
14$d2 = strtotime("2019-05-18 01:23:45");
15$totalSecondsDiff = abs($d1-$d2); //42600225
16$totalMinutesDiff = $totalSecondsDiff/60; //710003.75
17$totalHoursDiff   = $totalSecondsDiff/60/60;//11833.39
18$totalDaysDiff    = $totalSecondsDiff/60/60/24; //493.05
19$totalMonthsDiff  = $totalSecondsDiff/60/60/24/30; //16.43
20$totalYearsDiff   = $totalSecondsDiff/60/60/24/365; //1.35

1$date1 = "2007-03-24";
2$date2 = "2009-06-26";
3
4$diff = abs(strtotime($date2) - strtotime($date1));
5
6$years = floor($diff / (365*60*60*24));
7$months = floor(($diff - $years * 365*60*60*24) / (30*60*60*24));
8$days = floor(($diff - $years * 365*60*60*24 - $months*30*60*60*24)/ (60*60*24));
9
10printf("%d years, %d months, %d days\n", $years, $months, $days);

1function weeknumber($ddate) {
2    $date = new DateTime($ddate);
3
4    return $date->format('W');
5}