2018-09-03，注意时间。
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只有一个文件，代码：
<?php
if(!isset($_POST['n1'])){
$n1 = '';
$n2 = '';
$result = '';
$op = 'plus';
}else{
//接收数据
$n1 = $_POST['n1'];
$n2 = $_POST['n2'];
$op = $_POST['op'];
//根据接收到的运算符，进行不同的运算
switch($op){
case 'plus':
$result = $n1 + $n2;
break;
case 'minus':
$result = $n1 - $n2;
break;
case 'times':
$result = $n1 * $n2;
break;
case 'divide':
$result = $n1 / $n2;
break;
}
}
?>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1"/>
<title>calculator</title>
<meta name="description" content=""/>
<meta name="keywords" content=""/>
<link href="./bootstrap-3.3.7/dist/css/bootstrap.min.css" rel="stylesheet"/>
<script type="text/javascript" src="./bootstrap-3.3.7/dist/js/tests/vender/jquery.min.js"></script>
<script type="text/javascript" src="./bootstrap-3.3.7/dist/js/bootstrap.min.js"> </script>
</head>
<body>
<div class="container">
<form class="form-inline" action="calculator.php" method="post">
<h3 class="text-primary">网页版简易计算器</h3>
<div class="form-group">
<input class="form-control input-sm" type="text" name="n1" value="<?php echo isset($n1)?$n1:'';?>"/>
</div>
<select class="form-control input-sm" name="op"/>
<option value="plus" <?php echo $op=='plus'?'selected':'';?>>+</option>
<option value="minus" <?php echo $op=='minus'?'selected':'';?>>-</option>
<option value="times" <?php echo $op=='times'?'selected':'';?>>*</option>
<option value="divide" <?php echo $op=='divide'?'selected':'';?>>/</option>
</select>
<div class="form-group">
<input class="form-control input-sm" type="text" name="n2" value="<?php echo isset($n2)?$n2:'';?>"/>
</div>
<input class="btn btn-warning btn-sm" type="submit" value="="/>
<div class="form-group">
<input class="form-control input-sm" type="text" name="result" value="<?php echo isset($result)?$result:'';?>"/>
</div>
</form>
</div>
</body>
</html>