PHP matrix transpose using a functional approach

transpose.php

<?php
    $matrix = array(
        array('a', 1, 2),
        array('b', 3, 4),
        array('c', 5, 6),
        array('d', 7, 8)
    );

    $transpose = array_reduce(
        $matrix,
        function ($acum, $row) {
            array_walk(
                $row,
                function($column, $index, $acum) {
                    $acum[$index][] = $column;
                },
                &$acum
            );
            return $acum;
        },
        array()
    );

    print_r($transpose);

    // Output:
    // -------
    // a b c d
    // 1 3 5 7
    // 2 4 6 8
?>

// Taste it!
// php -f transpose.php

@qRoC can you explain how your beautiful trick works?