<?php
    $nums = "2 1 1 6 3";
    $numbers = explode(" ", $nums);
    $oddSum = 0;
    $evenSum = 0;
    $length = count($numbers);

    for ($i=0; $i < $length; $i++) {
        if ($i % 2 === 0) {
            $oddSum = $oddSum + $numbers[$i];
        }
        else {
            $evenSum = $evenSum + $numbers[$i];
        }
    }

    echo $oddSum;
?>