"Interleave" two strings in every possible way.

interleave.php

<?php
/*
 * "Interleave" two strings in every possible way.

 * interleave('ABC', 'DE') =
 * ABCDE ABDCE ABDEC ADBCE ADBEC ADEBC DABCE DABEC DAEBC DEABC
 *
 * Nothing real special here, just thought it was an interesting 
 * problem that gets annoying for humans but after some thought is
 * quite simple to a programmer and a computer.
 *
 * PHP isn't that horrible for functional-type programming either.
 */

function catter($a, $s) {
    return array_map(function($v) use ($s) { return $s . $v; }, $a);
}

function interleave($p, $q) {
    if (!strlen($p)) {
        return array($q);
    }
    if (!strlen($q)) {
        return array($p);
    }

    return array_merge(
            catter(interleave(substr($p, 1), $q), substr($p, 0, 1)),
            catter(interleave($p, substr($q, 1)), substr($q, 0, 1))
    );
}

$l = interleave('ABC', 'DE');
echo implode("\n", $l);