Geometric example

geometric.php

<?php

function sum($callback, $range) {
        return array_reduce($range, function($carry, $item) use ($callback) {
                return $carry + $callback($item);
        });
}

$test = function($x) { return (float) 1/$x; };
$y = sum($test, range(1, 3)); // 1/1 + 1/2 + 1/3 = 11/6
if ($y !== (float) 11/6) { exit('Fail!'); }

$funct = function($x) { return (float) 1/(2**$x); };
$cap = isset($argv[1]) ? $argv[1] : 10;
echo sum($funct, range(1, $cap));
echo PHP_EOL;