$text = 'Lorem Ipsum, is simply dummy text of, the printing and typesetting, industry. Lorem Ipsum, has been the industry, standard dummy, text ever since the 1500s, when an, unknown printer, took a galley of type and scrambled, it to make, a type specimen book.';

$search = ',';

$offset = 0;
$allpos = array();
while(($pos = strpos($text, $search, $offset)) !== FALSE){
    $offset = $pos + 1;
    $allpos[] = $pos;
}


if(count($allpos)>0){
    foreach($allpos as $k => $v){
        $k++;
        if($k % 5 == 0 && $k != 0){
            $text = substr_replace($text, '[,]', $v, strlen($search));
        }
    }
}

echo $text;