permutation efficiency

[boogyman]

I found a code that would create a permutation, however I was wondering if there was any way that I could make it more efficient?

Site: http://www.php.happycodings.com/Algorithms/code21.html

Code:


<?

function permutations($letters,$num){
$last = str_repeat($letters{0},$num);
$result = array();
while($last != str_repeat(lastchar($letters),$num)){
$result[] = $last;
$last = char_add($letters,$last,$num-1);
}
$result[] = $last;
return $result;
}

function char_add($digits,$string,$char){
if($string{$char} <> lastchar($digits)){
$string{$char} = $digits{strpos($digits,$string{$char})+1};
return $string;
}else{
$string = changeall($string,$digits{0},$char);
return char_add($digits,$string,$char-1);
}
}

function lastchar($string){
return $string{strlen($string)-1};
}

function changeall($string,$char,$start = 0,$end = 0){
if($end == 0) $end = strlen($string)-1;
for($i=$start;$i<=$end;$i++){
$string{$i} = $char;
}
return $string;
}

// define the parameters
$all = permutations('abcd',3);
$count = count($all);
// display the permutation
for($i=0; $i<$count; $i++) {
echo $all[$i] . "<BR>";
}

?>

[Nile]

In PHP 6, the <> is being removed (use !=).

And the {} array support is also being removed.