你只需要做一点加法：

$counter = 1; // set a counter
while($row = mysqli_fetch_assoc($result)){
    $fullname = $row['full_name'];
    $roll = $row['roll'];
    $s_result = $row['result'];
    $gm = $row['gm'];

    echo "<tr>";
    echo "<th scope='row'>$counter</th>"; // use the counter
    echo "<td>{$fullname}</td>";
    echo "<td>{$roll}</td>";
    echo "<td>{$s_result}</td>";
    echo "<td>$gm</td>";
    echo "</tr>";
    $counter++; // increase the counter
}

你需要在上面取一个全局变量 while 循环并将其初始化为 1 然后按原样打印并在每次迭代中增加，它将按序列号打印。

$index = 1;
while($row = mysqli_fetch_assoc($result)){
    $fullname = $row['full_name'];
    $roll = $row['roll'];
    $s_result = $row['result'];
    $gm = $row['gm'];

    echo "<tr>";
    echo "<th scope='row'>$index</th>";
    echo "<td>{$fullname}</td>";
    echo "<td>{$roll}</td>";
    echo "<td>{$s_result}</td>";
    echo "<td>$gm</td>";
    echo "</tr>";

    $index = $index + 1;
}