验证选择标签html php

Question

我正在尝试验证我的选择标记，以便在用户未选择会话时，按下提交按钮时将显示警告。我有我的html和php代码，谢谢

<select name="ses">

                <option> Select a Session </option>
                <?php

                $get_sess = "select * from sessions where course_id='1'";
                $run_sess = mysqli_query($con, $get_sess);

                while ($sess_row=mysqli_fetch_array($run_sess)){

                $ses_id=$sess_row['session_id'];
                $ses_title=$sess_row['session_title'];

                echo "<option value='$ses_id'>$ses_title</option>";
                } ?>
</select>

我的PHP代码是：

<?php
//if submit button is set i.e 'publish exercise now' pressed then:
if(isset($_POST['submit1'])){
    $exercises_ses = $_POST['ses'];
    $exercise_text = $_POST['exercise_text'];

        if($_POST['ses'] == 'NULL'){
            echo "<script>alert('Please select a session for the exercise')</script>";
        }
    }