Hitung Luas Bangun Data (part 2)

<!DOCTYPE html>
<html>
<head>
	<title>Menghitung Luas Persegi Panjang</title>
	<link rel="stylesheet" type="text/css" href="assets/css/bootstrap.css">
	<link rel="stylesheet" type="text/css" href="assets/css/bootstrap.min.css">
	<link rel="stylesheet" type="text/css" href="assets/css/bootstrap-grid.css">
	<script type="text/javascript" src="assets/js/bootstrap.min.js"></script>
	<script type="text/javascript" src="assets/js/bootstrap.js"></script>

</head>
<body>
	<div class="container">
		<div class="card">
    		<div class="card-header">
    				<h1>Menghitung Luas Persegi Panjang</h1>
    		</div>
    		<div class="card-body">
    			<div class="row">
    			<div class="col-md-4 col-sm-12">
    				<form action="./" method="post" enctype="multipart/form-data">
    				<div class="form-group">
    					<label>Panjang :</label>
    						<input type="text" name="panjang" class="form-control">
    				</div>
    				<div class="form-group">
    					<label>Lebar :</label>
    						<input type="text" name="lebar" class="form-control">
    				</div>
    				<div class="form-group">
    					<button type="submit" name="hitung" class="btn btn-success">Hitung</button>
    				</div>
    				</form>
    			</div>

    			<div class="col-md-4 col-sm-12">
    			</div>

			<?php if(isset($_POST['hitung']))
			{
			$panjang = $_POST['panjang'];
			$lebar = $_POST['lebar'];
			$luas;

			$luas = $panjang*$lebar; ?>
    			<div class="col-md-4 col-sm-12">

    				<div class="form-group">
    					<label>Luas :</label>
    						<input type="text" name="luas" class="form-control" value="<?php echo $luas ?>" disabled>
    				</div>

    			</div>
			<?php
			}
			?>
    			</div>
    		</div>
  		</div>
	</div>
</body>
</html>