Calendar month forward and month back

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PHP

Hello I'm trying to create my own calendar with just php, but I'm running into a problem. This is the top part of my calendar where I have 2 icons. one backwards and one forward arrow. just like in any calendar I want to be able to press those buttons and that the code will change the month to , in this case if you press the backward icon, April. The opposite for the forward arrow.

$sql = "SELECT * FROM reserveringen";
			$stmt = $conn->prepare($sql); 
			$stmt->execute();
			$result = $stmt->get_result();
			while ($row = $result->fetch_assoc()) {
				$start_datum = $row['start_datum'];
				$eind_datum = $row['eind_datum'];
				
				echo $start_datum;
				echo "<br>";
				echo $eind_datum;

			}
			// Get the current date
			// //dit hieronder is dus een array maar volgens mij kan het ook anders
			// $date = getdate();
			//print_r($date);

			//$date = date("d-m-Y");
			$date = new DateTime();

			//function for the buttons
			// if (isset($_POST['currentMonth'])) {
			// 	// kan zijn dat dit er nog in zou moeten $_POST['currentMonth']
			// 	$date = new DateTime();
			// }
			if (isset($_POST['monthBack'])) {
				$date = $date->add(new DateInterval('-1 month'));
			}
			else if (isset($_POST['monthFuture'])) {
				//add 1 month??
				$date = $date->add(new DateInterval('P1W2D'));
			}
			
			//date for above calender
			$calendarTiming = $date->format('d-m-Y');

			// functie voor de maand
			$currentMonth = $date->format('m');

			//check welk jaar het is
			$year = $date->format('Y');

			// hoeveel dagen zitten er in de maand
			$dagenCheck = cal_days_in_month(CAL_GREGORIAN, $currentMonth, $year);

			echo "<br>";

			echo "<thead>";
			echo "<tr>";
			echo "<th colspan='7'>
				<form method='POST' action=''>
				<input type='hidden' name='currentMonth' value='$currentMonth'>
					<button type='submit' name='monthBack'>
						" style="font-size: 20px">
					</button>";
						 print_r($calendarTiming);
						 echo " ". 
					"<button type='submit' name='monthFuture'>
						^__i class='fa fa-angle-right' style='font-size: 20px;'>
					</button>
				</form>
			</th>";
			echo "</tr>";
			echo "<br>";

but this code only works for one month back ward and/or forward.
so the only months I'm getting are 18-04-2022 , 18-05-2022 and 18-06-2022. But I want to be able to just scroll trough the months like and actual online calendar. is this possible in php?
it also doesn't update itself in this part

echo "<th colspan='7'>
				<form method='POST' action=''>
				<input type='hidden' name='currentMonth' value='$currentMonth'>
					<button type='submit' name='monthBack'>
						" style="font-size: 20px">
					</button>";
						 print_r($calendarTiming);
						 echo " ". 
					"<button type='submit' name='monthFuture'>
						^__i class='fa fa-angle-right' style='font-size: 20px;'>
					</button>
				</form>
			</th>";

it just keeps it the same date

What I have tried:

Tried to look it up on stack overflow but I got no results.
I also tried to put the if isset month back and the month future above the thead with the other date functions
but that only gives
18-04-2022 or 18-06-2022