$ _SESSION变量显示意外行为

Question

I'm currently working on a table with onclick() <td> elements that send info to a PHP page.

The basic idea is that if a course is clicked (egFrench, Maths, English, ...) the PHP page retrieves the value and afterwards executes the SQL code to give me all of the tutors for that specific course.

<script>
    function courseToTutor(id) {
        var courseClicked = document.getElementById("course" + id).value;
        $.post('loadDivs/loadTutors.php', {postname: courseClicked});
        $('#divTutors').load('loadDivs/loadTutors.php');
    }
</script>

When a <td> is clicked, it sends its id (generated in php) as a parameter. Inside the function I use that parameter to find the values within the <td> (stored inside an input type="hidden" ).

I have a single PHP page that both gets the posted value, like this:

if (isset($_POST['postname'])) {
    $course = $_POST['postname'];
    $_SESSION['clickedCourse'] = $course;
    echo $course;
}

The other part of the page is

else {
  //generate table with tutors for a specific course (with SQL)
}

This works about 85% of the time. Yes, I use correct session_start(); s, I have a proper database connection and so on.

When I click a <td> , I have an alert on the loadTutors.php that always triggers, it alerts my $_SESSION['clickedCourse]; .

85% of the time this alert show the correct info, the other 15% of the time, it is empty.

It's not my other code on the page, because I trigger the alert before I execute that code.

Thanks in advance

EDIT This is the php I use to generate the WHOLE table of courses.

<?php
$counter = 1;
if (!$res = $link->query($sqlSelectCourses)) {
    trigger_error('Error in query ' . $link->error);
} else {
    while ($row = $res->fetch_assoc()) {
        ?>
        <tr>
            <td onclick="courseToTutor(this.id);" id="<?php echo $counter; ?>">
                <?php echo $row["course"]; ?><input type="hidden" value="<?php echo $row["course"]; ?>" id="course<?php echo $counter; ?>">
            </td>
        </tr>
        <?php
        $counter+=1;
    }
}
?>

Answer 1

 $.post('loadDivs/loadTutors.php', {postname: courseClicked});
 $('#divTutors').load('loadDivs/loadTutors.php');

You are sending two requests, one to send data and one to receive new data. It can happen, that the second requests reaches the server first and i think this is what happens 15% the time (therefore wrong result). What you should do instead is using the first request and use its response:

 $.post('loadDivs/loadTutors.php', {postname: courseClicked}, function(response){
    $('#divTutors').html(response);
});

Edit: If you can't set your PHP-script up so it sends your new content directly at the "click-request", use this instead:

 $.post('loadDivs/loadTutors.php', {postname: courseClicked}, function(){
    $('#divTutors').load('loadDivs/loadTutors.php');
});

This will wait for the first request to finish before it starts the second.