writing code to sum things

PHP-Callable.php

<?php

function sum() {
    $list = array();
    $prepare = function($value) use(&$prepare, &$list) {
        if (is_array($value)) {
            return array_walk($value, $prepare, $list);
        } else {
            $list[] = $value;
        }
    };
        
    $args = func_get_args();
    array_walk($args, $prepare, $list);
    
    return addition(0, $list);
}

function addition($sum, array &$list) {
    return empty($list) ? $sum : addition($sum + array_pop($list), $list);
}

$vars = range(1, 1000);
echo sum($vars);