实现一个简单的计算器功能 -

实现一个简单的计算器功能

<!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>计算器</title>
    <style>
        body{
            background-color: #ccc;
        }
        h3{
            color: lightseagreen;
        }
    </style>
</head>
<body>
    <h3>计算器</h3>
    <form action="" method="post">
        <!-- !第一个数字 -->
        <input type="number" name="number1" value="<?=  isset($_POST['number1']) ? $_POST['number1'] : '' ?>">
        <select name="opt" id="">
            <option value="1" <?php echo isset($_POST['opt']) && $_POST['opt'] == '1' ? 'selected' : '' ?>>+</option>
            <option value="2" <?php echo isset($_POST['opt']) && $_POST['opt'] == '2' ? 'selected' : '' ?>>-</option>
            <option value="3" <?php echo isset($_POST['opt']) && $_POST['opt'] == '3' ? 'selected' : '' ?>>*</option>
            <option value="4" <?php echo isset($_POST['opt']) && $_POST['opt'] == '4' ? 'selected' : '' ?>>/</option>
            <option value="5" <?php echo isset($_POST['opt']) && $_POST['opt'] == '5' ? 'selected' : '' ?>>%</option>
        </select>
        <input type="number" name="number2" value="<?php echo isset($_POST['number2'])  ?>">
        <input type="submit" name="sub" value="计算">
    </form>
</body>
</html>
<?php
// echo $_POST['number1'];
$error = '';
if(($_POST['opt'] == 4 || $_POST['opt'] == 5) && $_POST['num2'] === 0)
{
       $error .= '除法运算的时候,第二个值不能为0';
}elseif(empty($_POST['number1']))
{
        $error .= '请输入第一个数!!!';
}elseif(empty($_POST['number2'])){
        $error .= '请输入第二个数!!!!';
}
if(isset($_POST['sub'])){
    if(empty($error)){
    switch($_POST['opt'])
        {
            case $_POST['opt'] == 1:
            $answer =  $_POST['number1'] + $_POST['number2'];
            break;
            case $_POST['opt'] == 2:
            $answer =  $_POST['number1'] - $_POST['number2'];
            break;
            case $_POST['opt'] == 3:
            $answer =  $_POST['number1'] * $_POST['number2'];
            break;
            case $_POST['opt'] == 4:
            $answer =  $_POST['number1'] / $_POST['number2'];
            break;
            case $_POST['opt'] == 5:
            $answer =  $_POST['number1'] % $_POST['number2'];
            break;
        }
        echo '您的答案:'.$answer;
    }else{
            echo $error;
}
}