php写一个计算器 -

<?php
    $result;
    $num1 = isset($_GET['num1']) ? $_GET['num1'] : 0;
    $num2 = isset($_GET['num2']) ? $_GET['num2'] : 0;
    $cal = isset($_GET['cal'])? $_GET['cal'] : 0;
    switch ($cal) {
        case '+':
            $result = $num1+$num2;
            break;
        case '-':
            $result = $num1-$num2;
            break;
        case '*':
            $result = $num1 * $num2;
            break;
        case '/':
            if ($num2 != 0) {
                $result = $num1 / $num2;
            } else {
                echo "被除数不可以为0";
            }
            break;
    }
?>
<!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>
</head>
<body>
    <h2>计算器</h2>
    <form action="" method="get">
        请输入第一个数<input type="text" name="num1"
            value="<?php echo $num1 ?>"><br>
        <input type="radio" name="cal" value="+" <?php if ($cal=="+") {
    echo "checked";
}?>>+
        <input type="radio" name="cal" value="-" <?php if ($cal=="-") {
    echo "checked";
}?>>-
        <input type="radio" name="cal" value="*" <?php if ($cal=="*") {
    echo "checked";
}?>>*
        <input type="radio" name="cal" value="/" <?php if ($cal=="/") {
    echo "checked";
}?>>/<br>
        请输入第二个数 <input type="text" name="num2"
            value="<?php echo $sum2 ?>">
        <br>
        <button>提交</button>
    </form>
    <?php
    echo "<h6>$num1 {$cal} $num2 = {$result}</h6>";
?>
</body>
</html>