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=$area
HTML; <div class="clear">
                 
              
              
        
            </div>