please help solve this problem, Fatal error: Uncaught Error: Call to a member function query() on null

please help me... Warning: Undefined variable $conn in C:\Users\ASUS\Documents\xammp\htdocs\intern\internstu\activity.php on line 51 and it said Fatal error: Uncaught Error: Call to a member function query() on null in C:\Users\ASUS\Documents\xammp\htdocs\intern\internstu\activity.php:51 Stack trace: #0 {main} thrown in C:\Users\ASUS\Documents\xammp\htdocs\intern\internstu\activity.php on line 51

This is my code

                        <tbody>
                        <?php
                        $act_query = $conn->query("SELECT * FROM `student_details`") or die($conn->error);
                        while($act_fetch = $act_query->fetch_array()){
                        ?>
                        <tr>
                            <td><?php echo $act_fetch['std_logbookActivite']?></td>
                            <td><?php echo "<label class = 'text-info'>".date("M d, Y", strtotime($act_fetch['std_logbookDate']))."</label>"?></td>
                            <td><center><a href = "edit_activity.php?detail_id=<?php echo $act_fetch['detail_id']?>" class = "btn btn-warning"><span class=  "glyphicon glyphicon-edit"></span> Update</a> <a href = "#" data-toggle = "modal" data-target = "#delete_activity" name = "<?php echo $act_fetch['detail_id']?>" class = "btn btn-danger detail_id"><span class=  "glyphicon glyphicon-trash"></span> Delete</a></center></td>
                        </tr>
                        <?php
                            }
                        ?>
                        
                        </tbody>

source https://stackoverflow.com/questions/67852895/please-help-solve-this-problem-fatal-error-uncaught-error-call-to-a-member-fu