Spiral Sort

spiral_sort.php

<?php

function spiral_sort($input)
{
    // hack to ensure $output has proper indexes. otherwise
    // resulting array has proper idx => value but may
    // actually be accessed incorrectly.
    $output = $input;

    $fromFront = 0;
    $fromBack = count($input) - 1;
    for ($i = 0; $i < count($input); $i++) {
        if (0 == $i % 2) {
            $output[$fromFront++] = $input[$i];
        } else {
            $output[$fromBack--] = $input[$i];
        }
    }

    return $output;
}

$input = array(1, 2, 3, 4, 5);

print_r($input);
print_r(spiral_sort($input));

/*

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
)
Array
(
    [0] => 1
    [1] => 3
    [2] => 5
    [3] => 4
    [4] => 2
)

Results without hack:

Array
(
    [0] => 1
    [4] => 2
    [1] => 3
    [3] => 4
    [2] => 5
)

*/