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<?php include("includes/css.php");
if(isset($_POST['submit'])){
extract($_POST);
date_default_timezone_get('Asia/Kolkata');
$date=date("Y-m-d H:i:s");
$id=$_GET['id'];
$ip=$_SERVER['REMOTE_ADDR'];
$sql ="UPDATE notification SET admin_subject='$subject',admin_message='$message',date='$date'  WHERE id='$id'"; 
$res=mysqli_query($conn,$sql);
if($res){
echo "<script>alert('Message updated successfully!!!'); 
                    location.replace('student-notification.php');
                </script>";  
}
}
$id=$_GET['id'];
$sql=mysqli_query($conn,"select * from notification where id='$id'");
$res= mysqli_fetch_array($sql);  ?>

<body class="hold-transition skin-blue sidebar-mini">
    <div class="wrapper">
        <?php include("includes/header.php");?>
        <?php include("includes/sidebar.php");?>
        
        <div class="content-wrapper">
            
            
            <section class="content">
                <div class="row">
                    <div class="col-md-12">
                        <div class="box box-primary">
                            <div class="box-header with-border">
                                <div class="col-md-7">
                                    <h3 class="box-title">Reply Student Notification:</h3>
                                </div>
                            </div>
                            
                            <div class="box-body"> 
                           <form action="" method="post">
                           <div class="form-group">
                              <input type="text" class="form-control" value=" <?php echo $res['admin_subject']; ?> " name="subject" placeholder="Subject">
                           </div>
                           <div>
                              <textarea class="textarea" name="message" placeholder="Message" style="width: 100%; height: 125px; font-size: 14px; line-height: 18px; border: 1px solid #dddddd; padding: 10px;"> <?php echo $res['admin_message']; ?> </textarea>
                           </div> 
                    
						<div class="col-md-3">
										<button type="submit" class="btn btn-success btn-lg mt-20" name="submit"><i class="fa fa-arrow-left"></i> Send</button> 
									</div>
								</form>
							</div> 
						</div>
					</div> 
			</div>
		</section>
    </div>
	<?php include("includes/footer.php");?>
	</div>
	<?php include("includes/js.php");?>
</body>

</html>

Editing File: reply-student-notification.php