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.