String Permutation Generator

permutations.php

<?php

// Needs php 5.6!
function combinations($length = 3, $chars = "abc")
{
    $p = array_fill(0, $length, 0); // counters
    $opt_count = strlen($chars);
    $iter_count = $opt_count ** $length;
    
    for($i = 0; $i < $iter_count; $i++) {
        // Build the code
        $s = '';
        for ($j = 0; $j < $length; $j++) {
            $s .= $chars[$p[$j]];
        }
        
        // offer to iterator
        yield $s;
        
        // advance the counters
        if ($i < $iter_count - 1) {
            $x = $length - 1;
            while($x >= 0) {
                $p[$x]++;
                if ($p[$x] == $opt_count) {
                    $p[$x] = 0;
                    $x--;
                } else {
                    break;
                }
            }
        }
    }
}

foreach(combinations(4, "PORK") as $x) {
    echo "$x\n"; // do the haxx
}