A QUICK IMPLEMENTATION OF STRING SORT IN PHP

fast-string-sort.php

<?php
function sortString(&amp;$s) {
	$s = str_split($s);
	sort($s);
	$s = implode($s);
}

// example usage prints:
// 1223344789aaaaadddefffffhhhiillllnnoorrsssuuuuwyy
$string = 'ouhlasfuywernhlasdfoulnarfiuyadf1234987234sdfailh';
sortString($string, strlen($string));
echo $string . PHP_EOL;

quicksort-implementation.php

<?php
function sortString($s) {
        $left = $right = '';
        $l = strlen($s) - 1;
        if ($l &lt;= 0) return $s;
        $pivot = floor($l/2);
        do {
                if ($l == $pivot) continue;
                if ($s[$l] &lt;= $s[$pivot]) $left .= $s[$l];
                else $right .= $s[$l];
        } while (--$l);
        return sortString($left) . $s[$pivot] . sortString($right);
}

// example usage prints:
// 1223344789aaaaadddefffffhhhiillllnnoorrsssuuuuwyy
$string = 'ouhlasfuywernhlasdfoulnarfiuyadf1234987234sdfailh';
$string = sortString($string);
echo $string . PHP_EOL;

slow-string-sort.php

<?php
/**
 * A (slow) non-array based solution for sorting strings in PHP.
 * 
 * @param	&$s	The string to be sorted
 * @param	$len	The length of the string
 * @param	$curPos	The current position being sorted (default = 0)
 */
function sortString(&$s, $len = 0, $curPos = 0) {
        if ($curPos === $len) return;
        $nextPos = $curPos + 1;
        while ($nextPos &lt; $len) {
                if ($s{$nextPos} &lt; $s{$curPos}) {
                        $tmp = $s{$curPos};
                        $s{$curPos} = $s{$nextPos};
                        $s{$nextPos} = $tmp;
                }
                ++$nextPos;
        }
        sortString($s, $len, $curPos + 1);
}

// example usage prints:
// 1223344789aaaaadddefffffhhhiillllnnoorrsssuuuuwyy
$string = 'ouhlasfuywernhlasdfoulnarfiuyadf1234987234sdfailh';
sortString($string, strlen($string));
echo $string . PHP_EOL;