<?php
mb_internal_encoding("UTF-8");

function doKstring($string, $s)
{
    
    $noAnsw  = "Sorry, your string is invalid\n";
    $letters = preg_split("//u", $string, null, PREG_SPLIT_NO_EMPTY);
    foreach ($letters as $number => $letter) {
        
        
        $key = array_keys($letters, $letter);
        if (count($key) != $s) {
            
            return $noAnsw;
            
        }
    }
    sort($letters);
    $word = $letters[0];
    for ($i = 1; $i <= $s; $i++) {
        
        $key = $i * $s;
        
        $word .= $letters[$key];
    }
    $newString = $word;
    for ($i = 1; $i <= $s; $i++) {
        $new_string .= $word;
    }
    
    return $new_string . "\n";
}
print_r(doKstring(авароыафвоыф, 4));
print_r(doKstring(абвавбабв, 3));
print_r(doKstring(ахкаахкхк, 3));
print_r(doKstring(авбубббуааввууав, 4));