How do I populate check boxes from Database using PHP/MySQLi - php

Question

I am working with three database tables.
users
class
user_class
I'm making a page so that student can be assigned to classes using a checkbox. How do I get it so that if the value of a student being in a class is currently in the database, the checkbox for that class will already have the student value checked.
<?php
$showAllStudents = "SELECT * FROM users";
mysqli_query($mysqli, $showAllStudents) or die ('Error finding Students');
$result = mysqli_query($mysqli, $showAllStudents);
echo"<table border='1' cellspacing='10' align='center'>";
echo "<tr><th></th><th>User ID</th><th>User Name</th><th>First Name</th>
<th>Second Name</th></tr>";
while ($row = $result->fetch_object()){
echo "<tr>";
echo "<td><input type='checkbox' id='" .$row->userID . "'
name='check_box[]' value='" .$row->userID . "' /></td>";
echo "<td>" .$row->userID . "</td>";
echo "<td>" .$row->username . "</td>";
echo "<td>" .$row->forename . "</td>";
echo "<td>" .$row->surname . "</td>";
echo "</tr>";
}
if (isset($_POST['submitClassStudent'])) {
//get ID from header
$classID = $_GET['id'];
//print_r ($_POST);
//for each ticked checkbox convert to a UserID variable
foreach ($_POST['check_box'] as $userID) {
$editClassStudentQuery = "INSERT INTO `user_class`(userID, classID)
VALUES('$userID', '$classID')";
$insert_row = $mysqli->query($editClassStudentQuery) or die($mysqli->error . __LINE__);
if ($insert_row) {
header('Location: classeditor.php');
} else {
echo "Error: " . $editClassStudentQuery . "<br>" . $mysqli->error;
}
}
}
?>

Answer 1

To mark a checkbox as already checked you just need to add checked as one of its attributes.
<input type="checkbox" id="already-checked" checked> Already checked
W3 Schools

Answer 2

if the user exists then echo 'checked' in tag
Example: echo "<input type ='checkbox'";if(YourConditionHere)echo "checked>";else echo ">";