Invalid argument when passing checkbox array from GET to POST - php

Question

PHP novice here. Goal is to:
a) Display a table of booking options (fitness classes) dynamically from database records, allow user to select multiple options with checkboxes against each row - this bit's working.
b) Pass the checkbox selection to a table listing the selected data on a confirmation page. I'm getting an error here: Invalid argument supplied for foreach().
c) Update the database when user hits the second page's 'Confirm' button.
Research so far uncovered this advice on using $_GET and $_POST to achieve this with an array.
My checkbox code on the initial page:
echo '<form action="makebooking.php" method="get">';
echo '<td><input type="checkbox" name="class_id[]" value=' . $row['class_id'] . '</td></tr>';
The foreach statement error comes from this code that generates the table of choices on the second page:
//Check if the GET is set from classes.php
if (isset($_GET['class_id'])) {
// Grab the score data from the GET
foreach($_GET['class_id'] as $class_id) {
$_GET['class_id'] = $class_id;
}
}
//table header
echo '<table class="table table-bordered table-hover">';
echo '<thead><tr><th>Date</th><th>Time</th><th>Venue</th><th>Who\'s going?</th> <th>Add someone</th></tr></thead>';
//create the form
echo '<form method="post" action="' . $_SERVER['PHP_SELF'] . '">';
//Get the class IDs from the GET to use in the POST
foreach ($_GET['class_id'] as $class_id) {
$sql = "SELECT class_id, DATE_FORMAT(date, '%a, %d %b') AS new_date, DATE_FORMAT(time, '%H:%i') AS new_time, venue FROM classes WHERE class_id = '$class_id'";
$data = mysqli_query($dbc, $sql);
//---------------------------------------------------------------------------------
//get table data
while ($row = mysqli_fetch_array($data)) {
$date = $row["new_date"];
$time = $row["new_time"];
$venue = $row["venue"];
$class_id = $row["class_id"];
}
//---------------------------------------------------------------------------------
//Show a table of the selected classes
echo '<input type="hidden" name="id" value= ' . $class_id . ' />';
echo '<td>' . $date . '</td>';
echo '<td>' . $time . '</td>';
echo '<td>' . $venue . '</td>';
echo '<td>' . $username . '</td>';
echo '<td><button class="btn btn-mini" type="button"><i class="icon-user"></i><i class="icon-plus"</i></button></td></tr>';
}
echo'</table>';
// Make booking button
echo '<input type="submit" name="submit" class="btn btn-large btn-primary pull-right" value="Confirm">';
echo '</form>';
}
Full code of both pages at this pastebin. All error-fixing advice gratefully accepted!

Answer 1

Figured out that I had been declaring the variables in if/else loops that made them inaccessible to other parts of the code.
I also added some extra hidden input arrays into both tables to validate the input. This required a string-to-integer conversion of the class ID in order to update the db with the selected data.
Full fixed code is here for anyone struggling with something similar.