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>