<h1>Список всех учителей</h1>
<p>Отобразить: <a href="april.php">Учителя с учениками рожденными в апреле</a>
                <a href="globalstudent.php">Учителя с максимальным кол-вом общих учеников</a>
</p>
<table border="1px">
    <tr>
        <th>ID</th>
        <th>Имя</th>
        <th>Пол</th>
        <th>Телефон</th>
        <th>Ученик/и</th>
    </tr>

        <?php
        $result = mysqli_query($link, "SELECT *, GROUP_CONCAT(student.sname ORDER BY student.sname ASC SEPARATOR ', ') AS sname, teacher.teacherid AS tid
                                        FROM teacher LEFT JOIN t_s ON teacher.teacherid = t_s.teacherid
                                        LEFT JOIN student ON t_s.studentid = student.studentid
                                        GROUP BY teacher.teacherid ASC");
        while ($row = mysqli_fetch_assoc($result)):
            $tid = $row['tid'];
            $tname = $row ['tname'];
            if ($row ['sex'] == 1) {
                    $tsex = "М";
                } else {
                    $tsex = "Ж";
                }
            $tphone = $row ['phone'];
            $sname = $row['sname'];

            ?>

    <tr>
        <td><?php echo $tid; ?></td>
        <td><?php echo $tname; ?></td>
        <td><?php echo $tsex; ?></td>
        <td><?php echo $tphone;?></td>
        <td><?php
                if (!empty($sname)) {
                          echo substr_count($sname, ',')+1;
                        } elseif (empty($sname)) {
                          echo "0";
                        }
         ?></td>
        <?php endwhile; ?>
    </tr>
</table>