<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<?php
$num = $_POST['num'];
$num1 = $_POST['num1'];
if(isset($_POST['btn'])){
$btn = $_POST['btn'];
$num *= 2;
$num1 = $num;
}else{
$num1 = $num;
};
?>
<form action="" method="post">
<input type="number" name="num" placeholder="մութքագրեք ձեր թիվը">
<input type="text" name='num1' value="ձեր թիվը <?php echo $num1?>" >
<input type="checkbox" name='btn'>
<button>Send</button>
</form>
</body>
</html>