test.php

<?php

$A = ["accaccqweara","khanhminh"];

function buildArraySortedByOccurrence($string)
{
    $data = [];
    $arrayN = str_split($string);
    foreach ($arrayN as $char) {
        if (!isset($data[$char])) {
            $data[$char] = 1;
        } else {
            $data[$char]++;
        }
    }
    arsort($data);
    return $data;
}

function buildArraySortedByAlphabetInTheSameOccurrence($array)
{
    $dataCount = [];
    foreach ($array as $alphabet => $count) {
        if (!isset($dataCount[$count])) {
            $dataCount[$count] = [$alphabet];
        } else {
            $merge = array_merge($dataCount[$count], [$alphabet]);
            asort($merge);
            $dataCount[$count] = $merge;
        }
    }
    return $dataCount;

}

function convertResultToString($dataCount)
{
    $result = -404;
    if (count($dataCount)) {
        $result = "";
        foreach ($dataCount as $count => $listChar) {
            foreach ($listChar as $char) {
                for ($i = 0; $i < $count; $i++) {
                    $result .= $char;
                }
            }
        }
    }
    return $result."\n";
}

foreach($A as $string){
    $array=buildArraySortedByOccurrence($string);
    $arrayDataCount=buildArraySortedByAlphabetInTheSameOccurrence($array);
    $result= convertResultToString($arrayDataCount);
    echo $result;
}