<?php

function combinations(array $pool, $r) {
    $n = count($pool);
    if ($r > $n) {
        return array();
    }
    $indices = range(0, $r - 1);
    $result = array();
    $tuple = array();
    foreach ($indices as $i) {
        $tuple[] = $pool[$i];
    }
    $result[] = $tuple;
    while (true) {
        $dup = true;
        for ($i = $r - 1; $i >= 0; --$i) {
            if ($indices[$i] != $i + $n - $r) {
                $dup = false;
            }
        }
        if ($dup) return $result;
        ++$indices[$i];
        for ($j = $i + 1; $j < $r; ++$j) {
            $indices[$j] = $indices[$j - 1] + 1;
        }
        $tuple = array();
        foreach ($indices as $i) {
            $tuple[] = $pool[$i];
        }
        $result[] = $tuple;
    }
}