PHP 函数用法

<?php
    
    $variable = ($_GET['modth'])? $_GET['modth']:'add';
    
    function add($firstval=0,$secondval=0){
        return $firstval+$secondval;
    }
    
    function subtract($firstval=0,$secondval=0){
        return $firstval-$secondval;
    }
    echo $variable(5,4);
?>