PHP实现计算器小功能

?php
 if (isset($_POST['sub'])) {
 $f = true;
 $error = "有以上问题: br>";
 if ($_POST["num1"] == "") {
  $f = false;
  $error .= "第一个数不能为空.br>";
 } else {
   if(!is_numeric($_POST[num1])) {
    $f = false;
    $error .= "第一个数字不是数字.br>";
   }
 }
 if ($_POST['num2'] == "") {
  $f = false;
  $error .= "第二个数字不能为空.br>";
 } else {
   if (!is_numeric($_POST['num2'])) {
   $f = false;
   $error .= "第二个数字不是数字br>";
  }
 }
 if ($f) {
  switch ($_POST['ysf']) {
  case '+': $sum = $_POST['num1']+$_POST['num2'];
   break;
  case '-': $sum = $_POST['num1']-$_POST['num2'];
   break;
  case '*': $sum = $_POST['num1']*$_POST['num2'];
   break;
  case '/': $sum = $_POST['num1']/$_POST['num2'];
   break;
  case '%': $sum = $_POST['num1']%$_POST['num2'];
   break;
  }
 }
 }
?>

!doctype html>
html>
head>
meta charset="utf-8">
title>简易计算器/title>
style>
 * {
  margin: 0px;
 padding: 0px;
 }
 #contains {
  width: 400px;
 margin: 50px auto;
 }
 h1 {
  width: 400px;
 text-align: center;
 color: #0F0;
 }
 table {
  margin-top: 20px;
 width: 400px;
 }
 input {
  background: yellow;
 border-radius: 20px;
 }
 input:hover {
  background: green;
 }
 select {
  background: yellow;
 border-radius: 20px;
 }
 td {
  background: #3F0;
 text-align: center;
 height: 30px;
 }
 #sub {
  border-radius: 25px;
 width: 50px;
 }
/style>
/head>
 
body>
 div id="contains">
  h1>简易计算器/h1>
 form action="Calculator.php" method="post">
 table border="0">
  tr>
  td>input type="text" name="num1" value="?php echo $_POST['num1'];?>" size="3"/>/td>
  td>
   select name="ysf">
   option ?php if($_POST['ysf'] == "+") echo "selected";?> value="+">+/option>
   option ?php if($_POST['ysf'] == "-") echo "selected";?> value="-">-/option>
   option ?php if($_POST['ysf'] == "*") echo "selected";?> value="*">*/option>
   option ?php if($_POST['ysf'] == "/") echo "selected";?> value="/">//option>
   option ?php if($_POST['ysf'] == "%") echo "selected";?> value="%">%/option>
   /select>
  /td>
  td>input type="text" name="num2" value="?php echo $_POST['num2'];?>" size="3"/>/td>
  td>input type="submit" id = 'sub' name="sub" value="计算"/>/td>
  /tr>
  tr>
   td colspan="4">
   ?php
    if ($f) {
    echo "计算结果: b>{$_POST['num1']}{$_POST['ysf']}{$_POST['num2']} = {$sum}/b>";
   } else {
    echo "b>$error/b>";
   }
   ?>
  /td>
  /tr>
 /table>
 /form>
 /div>
/body>
/html>