Using Function to Calculate Average

form.html
<html>
<head>
    <title>A form</title>
</head>
<body>
    <form method = "get" action = "test.php">
        WP:     <input type = "text" name = "wp" />
        CN:     <input type = "text" name = "cn" />
        COA:    <input type = "text" name = "coa" />
        <input type = "submit" value ="Find average"/>
    </form>
</body>
</html>

test.php
<html>
<body>
    <?php
        function calcAvg($wp, $cn, $coa){
            return ($wp+$cn+$coa)/3;
        }
        $wp = $_GET["wp"];
        $cn = $_GET["cn"];
        $coa = $_GET["coa"];
        $average = calcAvg($wp, $cn, $coa);
        echo "The average is ". $average;
    ?>
</body>
</html>