How can I limit or stop a form submission based upon a mysql count for a database drop-drop list?

Question

I have a career day registration system that allow a student to select 3 unique career choices and submit the results to a mysql database. However, I would like to limit the selections to 90 people per career choice.

Example career day schedule:

Session 1: Nursing (limit to 90) Session 2: Military (limit to 90) Session 3: Pharmacy (limit to 90)

Is it possible to do a mysql count and pass the count value to a javascript variable? Also If the count is > 90 a javascript validation happens and doesn't allow you to save to database.

<?php
$sql = "SELECT * FROM mytable WHERE session1 = 'Nursing';
$query = mysql_query($sql);
$num_rows = mysql_num_rows($query);
?>

<script>
var count = <?php echo $num_rows; ?>
function Form_Validator(editform)
{
if((editform.value_session2.value == "Nursing" && count > 90)) { alert("Nursing is closed!"); return (false); }

return (true);
}
</script>

I've tried this method in several different ways... but the values are not being passed. Please help!

Answer 1

You shouldn't be doing this in javascript as any client side script can be bypassed.

What you need to do is do a count on the number of students enrolled in that course before insertion into the database. If count >= 90 , then return an error saying the course is full. If count <= 90 , then proceed with a statement that inserts the data, then return a success message.

Answer 2

I understand this post is old, but here is what I did. When the user clicks on the Nursing session:

mysql_select_db($database_mySQLConnection, $mySQLConnection);
$query_registrant_count = sprintf("SELECT user_id FROM user_signup WHERE class_id = %s", GetSQLValueString($colname_registrant_count, "int"));
$registrant_count = mysql_query($query_registrant_count, $mySQLConnection) or die(mysql_error());
$row_registrant_count = mysql_fetch_assoc($registrant_count);
$totalRows_registrant_count = mysql_num_rows($registrant_count);

if ($totalRows_registrant_count > 19) {
    $returnToCourseList = "../courseListFileName.php";
   header(sprintf("Location: %s", $returnToCourseList . "?alert=" . urlencode("We're sorry, but this class is full. Please select another class or contact us for assistance.")));

exit; }

Then in courseListFileName.php, use Pekka's suggestion for Handling alerts in php

Answer 3

I would do a simple and structured algorithm to work this out:

1) During page load I would only show the avaliable options. Do a sql "group by", having all the "counts" and restrict the form appearence.

2) During the form submission check again about the possibility for a new applicant. Do a simple count again, check if it is possible to receive a new submission. That's it. This would save some concurrency problems and other means of bypassing.

This sounds more secure and clear.

Of course you would script a REST service to chek application avaiability during a form filling moment, but that is more work into something it could be simple.