这个是运用PHP的条件控制语句写的，比较简单，消化下...
<html> <head> <title>PHP实现简单计算器（使用分支结构）</title> </head> <body> <?php $mess = ""; if( isset( $_POST["sub"])){ if( $_POST["num1"] == ""){ $mess .= "第一个数不能为空！<br>"; } else if( !is_numeric($_POST["num1"])){ $mess .= "第一个数必须为数字！<br>"; } if( $_POST["num2"] == ""){ $mess .= "第二个数不能为空！<br>"; } else{ if( !is_numeric($_POST["num2"])){ $mess .= "第二个数必须为数字！<br>"; } else if( $_POST["opt"] == "/" && $_POST["num2"] == 0){ $mess .= "除数不能为0"; } } } ?> <table border="1" align="center" width="400"> <form action="" method="post"> <caption><h1>计算器</h1></caption> <tr> <td> <input type="text" size="4" name="num1" value="<?php echo $_POST["num1"] ?>" /> </td> <td> <select name="opt"> <option value="+"<?php echo $_POST["opt"] == "+" ? "selected" : "" ?>>+</option> <option value="-"<?php echo $_POST["opt"] == "-" ? "selected" : "" ?>>-</option> <option value="x"<?php echo $_POST["opt"] == "x" ? "selected" : "" ?>>x</option> <option value="/"<?php echo $_POST["opt"] == "-" ? "selected" : "" ?>>/</option> <option value="%"<?php echo $_POST["opt"] == "%" ? "selected" : "" ?>>%</option> </select> </td> <td> <input type="text" size="4" name="num2" value="<?php echo $_POST["num2"] ?>" /> </td> <td> <input type="submit" name="sub" value="计算" /> </td> </tr> </form> <?php if(isset($_POST["sub"])){ echo '<tr><td colspan="4">'; if( !$mess){ $sum = 0; switch($_POST["opt"]){ case "+": $sum = $_POST["num1"] + $_POST["num2"];break; case "-": $sum = $_POST["num1"] - $_POST["num2"];break; case "x": $sum = $_POST["num1"] * $_POST["num2"];break; case "/": $sum = $_POST["num1"] / $_POST["num2"];break; case "%": $sum = $_POST["num1"] % $_POST["num2"];break; } echo "结果: {$_POST['num1']} {$_POST['opt']} {$_POST['num2']} = {$sum}"; } else{ echo $mess; } echo '</td></tr>'; } ?> </table> </body> </html>