RecursiveSum.php

<?php

/**
 * Sum Interface
 *
 * @author Elliot Wright <elliot@byng.co>
 */
interface SumInterface
{
    /**
     * Sum a list of numbers
     *
     * @param array  $numbers
     * @param number $total
     *
     * @return number
     */
    public static function sum(array $numbers, $total = 0);
}

/**
 * Recursive Summing Class
 *
 * @author Elliot Wright <elliot@byng.co>
 */
class RecursiveSum implements SumInterface
{
    /**
     * {@inheritDoc}
     */
    public static function sum(array $numbers, $total = 0)
    {
        $total += array_pop($numbers);

        if (count($numbers) > 0) {
            return self::sum($numbers, $total);
        } else {
            return $total;
        }
    }
}

$numbers = [10, 20, 30, 40, 50];

$sum = RecursiveSum::sum($numbers);

var_dump($sum);