php

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
</head>
<body> <div class="col-md-6 ml-auto mr-auto mt-5">
<form method="POST" action="index.php">
  <div class="form-group">
    <label for="formGroupExampleInput">Input value</label>
    <input type="number" class="form-control" id="formGroupExampleInput" placeholder="Type input...." name="num1">
  </div>
  <div class="form-group">
    <label for="formGroupExampleInput2">Input value</label>
    <input type="number" class="form-control" id="formGroupExampleInput2" placeholder="Type input...." name="num2">
  </div>
	<div class="input-group mb-3">
  <div class="input-group-prepend">
    <label class="input-group-text" for="inputGroupSelect01">Operator Options</label>
  </div>
  <select class="custom-select" id="inputGroupSelect01" name="ope">
    <option selected>Choose...</option>
    <option value="1">+</option>
    <option value="2">-</option>
    <option value="3">*</option>
    <option value="3">/</option>
  </select>
</div>

	<button class="btn btn-primary" type="submit">Submit</button>
</form>


</div>


<?php

$num1= $_POST["num1"];
$num2= $_POST["num2"];
$ope= $_POST["ope"];

if ($ope=="1") {
	$total=$num1+$num2;
}elseif ($ope=="2") {
	$total=$num1-$num2;
}elseif ($ope=="3") {
	$total=$num1*$num2;
}elseif ($ope=="4") {
	$total=$num1/$num2;
}

 ?>
 <div class="col-md-6 ml-auto mr-auto mt-5">
 <label for="formGroupExampleInput2"><b>Answer:</b></label>
    <input readonly type="text" class="form-control" id="formGroupExampleInput2" placeholder="readonly type..." value='<?php echo "$total";  ?>'>
</div>


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</body>
</html>