How to dynamically display links in web pages using PHP - php

Question

Good day # all.
I've this code snippet which's aim is to display the Exam this user is qualified to take based on the courses registered for. It would display the Exam Name, Date Available, Passing Grade and either Take Exam link if he/she hasn't written or View Result if he/she has written previously.
/*Connection String */
global $con;
$user_id = $_SESSION['user_id']; //user id
$courses = parse_course($user_id); //parse course gets the list of registered courses (Course Codes) in an array
foreach ($courses as $list)
{
$written = false;
$list = parse_course_id($list); //parse_course_id gets the id for each course
$ers = mysqli_query($con, "Select * from exams where course_id = '$list'");
while ($erows = mysqli_fetch_assoc($ers)) {
$trs = mysqli_query($con, "Select * from result_data where user_id = '$user_id'");
while ($trows = mysqli_fetch_assoc($trs)) {
if ($trows['user_id'] == $user_id && $trows['exam_id'] == $erows['exam_id'])
$written = true;
else
$written = false;
}
if($written)
{
echo "<tr><td>".$erows['exam_name']."</td><td>".$erows['exam_from']." To ".$erows['exam_to']."</td><td>".$erows['passing_grade']."%</td><td>".'View Result '."</td></tr>";
$written = false;
}
else
{
echo "<tr><td>".$erows['exam_name']."</td><td>".$erows['exam_from']." To ".$erows['exam_to']."</td><td>".$erows['passing_grade']."%</td><td>".'Take Exam '."</td></tr>";
$written = false;
}
}
}
But It only displays one View Result entry even if I've taken more than one exam. It shows the recent entry. Please what am I missing?

Answer 1

Untested, but here's how I would do it.
I've assumed $user_id is an integer. I'm a bit worried about it being used in SQL without any sanitization. I can't guarantee anything else you're doing is secure either because I can't see your other code. Please read: http://php.net/manual/en/security.database.sql-injection.php
(Oh I see someone already commented on that - don't take it lightly!)
Anyway, my approach would be to collect the user's written exam IDs into an array first. Then loop through the available exams and check each exam id to see if it's in the array we made earlier.
I wouldn't bother looking into the join advice unless you find this is performing poorly. In many systems it would be common to have 3 functions in this situation, one that generates $users_written_exam_ids ones that pulls up something like $all_available_exams and then this code which compares the two. But because people are seeing both queries here together there is a strong temptation to optimize it, which is cool but you probably just want it to work :)
<?php
global $con;
// Get the user id. Pass through intval() so no SQL injection is possible.
$user_id = intval($_SESSION['user_id']);
// Parse course gets the list of registered courses (Course Codes) in an array
$courses = parse_course($user_id);
foreach ($courses as $list)
{
// Gets the id for each course
$list = parse_course_id($list);
$users_written_exam_ids = array();
$trs = mysqli_query($con, "SELECT exam_id FROM result_data WHERE user_id = '$user_id'");
while ($trows = mysqli_fetch_assoc($trs))
{
$users_written_exam_ids[] = $trows['exam_id'];
}
$ers = mysqli_query($con, "SELECT * FROM exams WHERE course_id = '$list'");
while ($erows = mysqli_fetch_assoc($ers)) {
echo '<tr><td>' . $erows['exam_name'] . '</td><td>' . $erows['exam_from']
. ' To ' . $erows['exam_to'] . '</td><td>' . $erows['passing_grade']
. '%</td><td>';
if (in_array($erows['exam_id'], $users_written_exam_ids))
{
echo 'View Result';
}
else
{
echo 'Take Exam';
}
echo '</td></tr>';
}
}