How to display while loop counter variable

You can print $no variable value:

<table id = "result" class="data-table">
    <caption class="title"></caption>
    <thead>
        <tr>    

            <th>Sr.No.</th>
            <th>Student ID</th>
            <th>Student Name</th>
            <th>Marks Obtained</th>
        </tr>
    </thead>
    <tbody>

    <?php
    $no     = 1;
    $total  = 0;
    while ($row = mysqli_fetch_array($query))
    {
        $stu  = $row['stu_id'] == 0 ? '' : number_format($row['stu_id']);
        echo '<tr>


                <td>'.$no.'</td>
                <td>'.$row['student_id'].'</td>
                <input type="hidden" name="student_id" value='.$row['student_id'].'>
                <td>'.$row['student_name'].'</td>
                <input type="hidden" name="student_name" value='.$row['student_name'].'>
                <td>'."<div class='search-block clearfix'><input name='obtmarks' placeholder='' type='number'></div>".'</td>

            </tr>';
        $total += $row['stu_id'];

        $no++;


    }?>
    </tbody>

</table>

@Akhtar, I understand you are quite new to the PHP world. All the previous answers you have received are already right, but I would like to contribute in your question with some small styles suggestions that will help you writing good code and finding eventual issues by yourself.

Changes:

• general variables moved to the top
• PHP is not more printing HTML, but just contents
• merged all the new variables in a single block ($total and $stu)
• commented not used variable (may be you will use it in future development)
• input elements moved into the td (there should not be code between cells)
• uniformed quotes (') and double-quotes ("), HTML with double quotes, PHP with quotes
• changed the while parentheses with a clearer syntax (you will find it useful with long HTML)

With these changes, if you are going to use a modern IDE, you will have better suggestions and the code highlight will help you.

New Code:

<?php
$no    = 0;
$total = 0;
?>
<table id="result" class="data-table">
    <caption class="title"></caption>
    <thead>
        <tr>    
            <th>Sr.No.</th>
            <th>Student ID</th>
            <th>Student Name</th>
            <th>Marks Obtained</th>
        </tr>
    </thead>

    <tbody>
    <?php while ($row = mysqli_fetch_array($query)): ?>
        <?php
        $total += $row['stu_id'];

        //aren't you using this variable?
        //$stu = $row['stu_id'] == 0 ? '' : number_format($row['stu_id']);
        ?>
        <tr>
            <td><?php echo ++$no ?></td>
            <td><?php echo $row['student_id'] ?></td>
            <td><?php echo $row['student_name'] ?></td>
            <td>
                <input type="hidden" name="student_id" value="<?php echo $row['student_id'] ?>">
                <input type="hidden" name="student_name" value="<?php echo $row['student_name'] ?>">
                <div class="search-block clearfix">
                    <input name="obtmarks" placeholder="" type="number">
                </div>
            </td>
        </tr>
    <?php endwhile; ?>
    </tbody>
</table>