344. Reverse String

class Solution {

    /**
     * @param String[] $s
     * @return NULL
     */
    function reverseString(&$s) {
        $start = 0;
        $end = array_key_last($s);
        while($start < $end) {
            $startTemp = $s[$start];
            $endTemp = $s[$end];
            $s[$start++] = $endTemp;
            $s[$end--] = $startTemp;
        }

        return 'easy!';
    }
}