sporadic field updates with InnoDB

[Juergen Ahlmann]

I have an educational course. Every time the student finishes a lesson, I update a field to keep track of where the student has to go when he logs-in next time. Sometimes this works well lesson after lesson. Other times the field simply stops updating even though the student may have gone through many more lessons. It is also unpredictable when the field stops updating. Here is the simple code:

<?php
$nextLesson = "LS03Z";
$LoginID = $_COOKIE['dmv_cookie'];
require_once('../includes/conndb.inc');
require_once('../includes/sestim.inc');
$sql = "UPDATE ststats
SET U_gotoLesson = '$nextLesson'
WHERE U_StudentLogin = '$LoginID'";
$result = mysql_query($sql);
if (!$result) {
die('Could not Update: ' . mysql_error());
// echo "Could not update ststats ($sql) from DB: " . mysql_error();
// echo "<h2>Could not update ststats. Error Code S0903</h2>";
// echo "<h2>Please note the Error Code and inform your Driving School</h2>";
mysql_close($conn);
exit;
}
mysql_close($conn);
header("Location: ../lss8wqreph7490dp4zx8f/LS03ZPOa86akL4/player.html");
?>

Does anybody have any ideas?

Juergen R. Ahlmann
Mexifornia