【解決方法】エラー: C:\wamp64\www\online-exam-portal\code\manage-exam\_export-to-csv.php の 85 行目で非オブジェクトのプロパティ ‘num_rows’ を取得しようとしています

DB からすべての受験生に関する情報を取得し、データを CSV にエクスポートしようとしていますが、このエラーが発生します。

<tr><th align='left' bgcolor='#f57900' colspan="5"><span style='background-color: #cc0000; color: #fce94f; font-size: x-large;'>( ! )</span> Notice: Trying to get property 'num_rows' of non-object in C:\wamp64\www\online-exam-portal\code\manage-exam\_export-to-csv.php on line 85</th></tr>

これが私が書いたコードです。export-to-csv.php でデータを取得します。

  1  <?php
  2  require_once '../connect.php';
  3  require_once '../session.php';
  4  
  5  ini_set('display_errors', 1);
  6  ini_set('display_startup_errors', 1);
  7  error_reporting(E_ALL);
  8  
  9  // Check if the user is logged in
 10   //(!isset($_SESSION['user_id'])) {
 11      //Redirect or handle unauthorized access
 12      //header('Location: ../session.php')
 13     //exit();
 14  //}
 15  
 16  // Set the content-type to CSV and specify the filename for download
 17  header("Content-type: application/csv");
 18  header("Content-Disposition: attachment; filename=user_responses.csv");
 19  header("Pragma: no-cache");
 20  header("Expires: 0");
 21  
 22  // Get the user ID from the session
 23  $userID = $_SESSION['userID'];
 24  $examID = $_POST['examID'];
 25  
 26  //echo "<pre>" . $userID . "<pre/>";
 27  //exit();
 28  
 29  // Fetch user responses to questions
 30  $sql = "SELECT
 31              'Multi-choice' AS exam_type,
 32              mcq.question AS question_text,
 33              mcq.correct_answer AS correct_answer,
 34              mcqr.response AS chosen_option,
 35              mcqr.score AS marks
 36          FROM
 37              multi_choice_question mcq
 38              INNER JOIN multi_choice_response mcqr ON mcq.exam_id = mcqr.exam_id
 39                  AND mcq.question_no = mcqr.question_no
 40          WHERE
 41              mcqr.assignee_id = '$userID'
 42              AND mcq.exam_id = $examID
 43  
 44          UNION
 45  
 46          SELECT
 47              'Fill in the blank' AS exam_type,
 48              fiq.question AS question_text,
 49              fir.response AS chosen_option,
 50              fir.score AS marks
 51          FROM
 52              fill_in_question fiq
 53              INNER JOIN fill_in_response fir ON fiq.exam_id = fir.exam_id
 54                  AND fiq.question_no = fir.question_no
 55          WHERE
 56              fir.assignee_id = '$userID'
 57              AND fiq.exam_id = $examID
 58  
 59          UNION
 60  
 61          SELECT
 62              'Theory' AS exam_type,
 63              tq.question AS question_text,
 64              tr.response AS chosen_option,
 65              tr.score AS marks
 66          FROM
 67              theory_question tq
 68              INNER JOIN theory_response tr ON tq.exam_id = tr.exam_id
 69                  AND tq.question_no = tr.question_no
 70          WHERE
 71              tr.assignee_id = '$userID'
 72              AND tq.exam_id = $examID";
 73  
 74  $result = mysqli_query($conn, $sql);
 75  //echo "<pre>" . $result . "<pre/>";
 76  //exit();
 77  
 78  // Create the output file (without saving it)
 79  
 80  $dataFile = fopen('php://output', 'w');
 81  
 82  
 83  
 84  
 85  if ($result->num_rows > 0) {
 86  
 87      //if ($result) {
 88        //  if ($result ->num_rows > 0) {
 89      
 90  
 91  // Get the total possible mark
 92  $markResult = $conn->query("SELECT total_mark FROM exam WHERE exam_id = $examID");
 93  $totalMark = $markResult->fetch_assoc()['total_mark'];
 94  
 95    // Output the data
 96    while ($row = $result->fetch_assoc()) {
 97      fputcsv($dataFile, $row);
 98  // }
 99  }
100  
101      // Output the header
102      fputcsv($dataFile, array("QUESTION TYPE", "QUESTION", "CORRECT ANSWER", "CHOSEN OPTION", "SCORE(out of $totalMarks)"));
103  
104  }
105      else {
106      echo "0 results";
107  }
108  
109  // Close the file handle
110  fclose($dataFile);
111  
112  // Close the database connection
113  mysqli_close($conn);
114  ?>

私が試したこと：

C:\wamp64 の非オブジェクトのプロパティ ‘num_rows’ を取得しようとしています