thôi tôi giải đc rồi
function findFirstRepeated($inputString){
    $charArr = str_split($inputString);
    $firstRepeatedIndex = findFirstCharRepeated($inputString);


    if($firstRepeatedIndex == -1 ){
       return "";
    }

    $text = $charArr[$firstRepeatedIndex];

    $output = $charArr[$firstRepeatedIndex] . "";


    for($i = $firstRepeatedIndex + 1 ; $i <count($charArr); $i ++){
       
            if($charArr[$firstRepeatedIndex] == $charArr[$i]){
                $j = 1;
                while($j + $firstRepeatedIndex < $i  && $i + $j < count($charArr)){
                    if($charArr[$firstRepeatedIndex + $j] == $charArr[$j +$i]){
                        $output .= $charArr[$j +$i];
                        var_dump($charArr[$j +$i]);
                    }else{
                        return $output;
                    }
                    $j++;
                }
                return $output;
            }
        
    }
    return $output;
}


function findFirstCharRepeated($input){
    $input = str_split($input);
    for($i = 1 ; $i <count($input); $i++){
        for($j = 0 ; $j < $i ; $j ++){
            if($input[$i] == $input[$j]){
                return $j;
            }
        }
    }
    return -1;   
}