validate select tag html php - php

Question

I'm trying to validate my select tag so that if the user has not selected a session an alert will appear when the submit button is pressed. I have my html and php code below, thanks
<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>
my php code is:
<?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>";
}
}

Answer 1

You should just use the empty function. This will check that the value isn't "empty", check the manual for everything that qualifies as "empty", per comments I think this meets your criteria.
if(isset($_POST['submit1'])){
$exercises_ses = $_POST['ses'];
$exercise_text = $_POST['exercise_text'];
if(empty($_POST['ses'])){
echo "<script>alert('Please select a session for the exercise')</script>";
} else {
// execute functions form value is correct
}
}

Answer 2

'NULL' is a string, if you want to compare to null value - use NULL.
I don't think that $ses_id in your db has value 'NULL'.
Checking if value is set can be done with isset, and with empty for not empty:
if (isset($_POST['ses']) && !empty($_POST['ses'])) {