PHP解法
<html>
<head>
<meta charset="utf-8">
<title>A1</title>
</head>
<body>
<div>
<form method="post">
<div>
<select name="select1" id="select1">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
</div>
<div>
<input type="submit" value="copy">
</div>
</form>
<div>
<textarea><?=isset($_POST['select1']) ? $_POST['select1'] : ''; ?></textarea>
</div>
</div>
</body>
</html>
JQuery解法
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>A2</title>
</head>
<body>
<div>
<div>
<select name="select1" id="select1">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
</div>
<div>
<input type="button" id="copy" value="copy">
</div>
<div>
<textarea id="show"></textarea>
</div>
</div>
</body>
<script src="../public/js/jquery-3.2.0.min.js"></script>
<script>
$(function () {
$('#copy').click(function(){
$('#show').text($('#select1').val());
});
});
</script>
</html>