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

------解决方案--------------------

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><br><font color="#e78608">------解决方案--------------------</font><br>

PHP code

$area = 0;if(isset($_GET['r'])) $area = pi()*$_GET['r']*$_GET['r'];echo 半径<input type="text" name="r"><input type="submit" value="计算">面积=$areaHTML;<div class="clear">
                 
              
              
        
            </div>