<?php
    $connection = mysqli_connect('localhost', 'root', '', 'test');
    mysqli_set_charset($connection, 'utf8');
    if (!$connection) {
        die("Database connection failed: " . mysqli_error());
    }
    $sql = "SELECT QuestionHeader, QuestionText, QuestionVotes FROM question ORDER BY QuestionVotes DESC LIMIT 3";
    $result = $connection->query($sql);

    if ($result->num_rows > 0) {
         // output data of each row
         while($row = $result->fetch_assoc()) {
             echo "<div class="col-md-4"><h2>". $row["QuestionHeader"]. "</h2><p>". $row["QuestionText"]. "</p><p><a class="btn btn-success"> " . $row["QuestionVotes"] . "</a></p></div>";
         }
    } else {
         echo "0 results";
    }

    $connection->close();
?>

$dbname = "DB HERE";
 $servername = "HOST HERE";
 $username = "DB USER HERE";
 $password = "DB PASSWORD HERE";

  // Create connection
  $conn = mysqli_connect($servername, $username, $password, $dbname);

                 if(isset($_GET['id']))
            {
                ///Check to see if user already voted
                $result = $conn->query("SELECT * FROM User_Votes where user id = $session_id and question_id = $id");
                $row_cnt = $result->num_rows;

                if($row_cnt < 1)
                {
                    ///SQL to insert vote into Users Votes table
                }else
                {
                    //Vote already exists
                }

            }

            // Loop through questions for voting
            $result = mysqli_query($conn,"select * from questions");
            while($db_questions = mysqli_fetch_object($result))
            {
                echo $db_questions->question_title;
                echo '- <a href="mypage.php?id=$db_questions->question_id">Click to Vote</a>;
            }

<?php
// Need to assign the user's ID to a variable ($userID) to pass to the form.
$userID = '123'; // this needs to be handled on your end.

// updated sql to include Id and voters
$sql = "SELECT QuestionID, QuestionHeader, QuestionText, QuestionVotes, QuestionVoters FROM question ORDER BY QuestionVotes DESC LIMIT 3";

while($row = $result->fetch_assoc()) {

    $voters = json_decode($row['QuestionVoters'], true); // array of userid's that have voted
    IF (in_array($userID, $voters)) {
        // user has voted
        echo "n
        <div class="col-md-4">
            <h2>". $row["QuestionHeader"]. "</h2>
            <p>". $row["QuestionText"]. "</p>
            <p>" . $row["QuestionVotes"] . "</p>
        </div>";
    }ELSE{
        // user has not voted
        echo "n
        <div class="col-md-4">
            <form action="vote_processing.php" name="voting" method="post">
            <input type="hidden" name="qid" value="".$row['QuestionID']."" />
            <input type="hidden" name="userid" value="".$userID."" />
            <h2>". $row["QuestionHeader"]. "</h2>
            <p>". $row["QuestionText"]. "</p>
            <p><button type="submit" value="Submit">" . $row["QuestionVotes"] . "</button></p>
            </form>
        </div>";
    }

}
?>

<?php
IF (isset($_POST['qid'])) {

    $qid = htmlspecialchars(strip_tags(trim($_POST['qid']))); // basic sanitization
    $userid = htmlspecialchars(strip_tags(trim($_POST['userid']))); // basic sanitization

    IF ( (is_int($qid)) && (is_int($userid)) ) { // validate that both are integers

        // db connection
        $connection = mysqli_connect('localhost', 'root', '', 'test');
        mysqli_set_charset($connection, 'utf8');
        if (!$connection) {
            die("Database connection failed: " . mysqli_error());
        }

        // Get voters array
        $sql = "SELECT QuestionVoters FROM question WHERE QuestionID = '".$qid."'";
        $result = $connection->query($sql);
        if ($result->num_rows > 0) {
            while($row = $result->fetch_assoc()) {
                IF (!empty($row['QuestionVoters'])) {
                  // decode users array
                  $voters = json_decode($row['QuestionVoters'], true);
                }ELSE{
                  $voters = array(); // create array
                }
            }
            mysqli_free_result($result);

            // re-validate the userID "is not" in array
            IF (!in_array($userid, $voters)) { // note the ! [meaning NOT].

                $voters[] = $userid; // add userid to voters array
                $qvoters = json_encode($voters); // encode voters array

                // update vote
                $sql_upd = "UPDATE question SET QuestionVotes = QuestionVotes + 1, QuestionVoters = $qvoters WHERE QuestionID = '".$qid."'";
                $upd_result = $connection->query($sql_upd);

            }

        }

        mysqli_close($connection);

    }

}

// redirct back to previous page
?>