求一段用php代码,该怎么解决

求一段用php代码
求一段用php代码，网页上有个输入框，输入半径之后，点击按钮求圆的面积

[解决办法]

PHP code

<?php    echo "<input type='text' id='round' value=''/>";    echo "<input type='button' id='submit' value='Submit' onclick='javascript:getArea()'>";    echo "<p id='area'></p>";?>    <script>    var round;    var area;    var PI = 3.14;    function getArea()    {        round = parseFloat(document.getElementById("round").value);        area = 4/3*PI*round*round;        setArea(area);    }    function setArea(area)    {        document.getElementById("area").innerHTML = "The Round's Area is : "+area ;    }</script>
[解决办法]
PHP code
$area = 0;if(isset($_GET['r'])) $area = pi()*$_GET['r']*$_GET['r'];echo <<< HTML<form>半径<input type=text name=r><input type=submit value=计算></form>面积=$areaHTML;