eval("\$str = $num1 $operate $num2;");
注：1. eval（）中必须使用双引号
2. 函数中必须加英文状态下的分号
<!DOCTYPE html>
<html>
<head>
<meta charset='utf-8'>
<title>
</title>
</head>
<body>
<form action="" method="get" accept-charset="utf-8">
<input type="text" name="num1" value="" placeholder="">
<select name="operate" >
<option value="+">加</option>
<option value="-">减</option>
<option value="*">乘</option>
<option value="/">除</option>
</select>
<input type="text" name='num2'>
<input type="submit" value="提交">
</form>
<?php
if(!empty($_GET['num1'])&&!empty($_GET['num2'])&&!empty($_GET['operate'])){
// switch ($_GET['operate']) {
// case "+":
// $str=$_GET['num1']+$_GET['num2'];
// break;
// case "-":
// $str=$_GET['num1']-$_GET['num2'];
// break;
// case '*':
// $str=$_GET['num1']*$_GET['num2'];
// break;
// case "/":
// $str=$_GET['num1']/$_GET['num2'];
// break;
// default:
// $str='输入的数不合法！';
// break;
// }
// $str=eval("$_GET['num1'] $_GET['operate'] $_GET['num2']");
$a=$_GET['num1'];
$b=$_GET['operate'];
$c=$_GET['num2'];
eval("\$str = $a $b $c;");
echo "{$_GET['num1']} {$_GET['operate']} {$_GET['num2']} = ". $str;
}else{
echo "请输入要计算的数和计算方式！";
}
?>
</body>
</html>