Quiz Maker SQLi

//quiz-maker.8.8.0/quiz-maker/admin/class-quiz-maker-admin.php
...
    public function ays_questions_statistics_export() {
        error_reporting(0);
        global $wpdb;

        $quiz_id = isset($_REQUEST['id']) ? $_REQUEST['id'] : null;
        $quizes_table = $wpdb->prefix . 'aysquiz_quizes';
        $quizes_questions_table = $wpdb->prefix . 'aysquiz_questions';
        $sql = "SELECT question_ids FROM {$quizes_table} WHERE id = {$quiz_id};";
        $results = $wpdb->get_var( $sql);
        $questions_ids = array();
        $questions_counts = array();
        $questions_list = array();
        if($results != ''){
            $results = explode("," , $results);
            foreach ($results as $key){
                $questions_ids[$key] = 0;
                $questions_counts[$key] = 0;
                $sql = "SELECT question FROM {$quizes_questions_table} WHERE id = {$key} ; ";
                $questions_list[$key] = $wpdb->get_var( $sql);
            }
        }
...
        $quests = array();
        $questions_headers = array(
            array( 'text' => "question" ),
            array( 'text' => "correctness" ),
            array( 'text' => "correct_answers" ),
            array( 'text' => "answered_questions" )
        );
        $export_data = array();
        $quests[] = $questions_headers;

        foreach ($questions_ids as $n => $a){
            if ($a != 0 ||  $questions_counts[$n] != 0){
                $score = round($a/$questions_counts[$n]*100, 1) . "%";
            }else {
                $score = 0 . "%";
            }

            $q = array(
                array( 'text' => strip_tags(stripslashes(str_replace("\n", "", wpautop($questions_list[$n])))) ),
                array( 'text' => $score ),
                array( 'text' => $a ),
                array( 'text' => $questions_counts[$n] )
            );

            $quests[] = $q;
        }

        $export_data = array(
            'status' => true,
            'type'   => 'xlsx',
            'data'   => $quests
        );

        ob_end_clean();
        $ob_get_clean = ob_get_clean();
        echo json_encode($export_data);
        wp_die();
    }
...