Display in a php table foreign Keys from Mysql

Question

I have the following tables in my database:

CREATE TABLE subjects (
  subject_id int(11) NOT NULL AUTO_INCREMENT,
  subject_name text,
  PRIMARY KEY (subject_id)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=1;

CREATE TABLE users (
  id int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY,
  username varchar(100) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE users_subjects (
  users_subjects_id int(11) NOT NULL AUTO_INCREMENT,
  user_id_fk int(11),
  subject_id_fk int(11),
  FOREIGN KEY(user_id_fk) REFERENCES users(id),
  FOREIGN KEY(subject_id_fk) REFERENCES subjects(subject_id),
  PRIMARY KEY (users_subjects_id)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=1;

In the table 'users_subjects' I’m trying to relate the 'subjects' and 'users' tables. All the data in the tables are entered from my index.php.

I introduce the subject_name from my index.php and every time I enter a new one, checkboxes like these are created in the part where the user is added:

In this way, when entering a user and choosing the subjects that are desired, the 'users_subjects' table is created.

If for example I enter a user 'EXAMPLE' with ID = 1 and the SAT and ICT subjects with IDs 1 and 2, the 'users_subjects' table would look something like this:

The problem is that I am trying to show the users with their corresponding subjects in a PHP table but I doN’t know how to show the subjects separated by commas in the column 'SUBJECT'. I would like to do something like the following image but I don’t know how to do it. Can somebody help me? Thank you

This is my index.php:

<table class="users">
    <tr>
        <th colspan="2"><h2>Users registered</h2></th>
    </tr>
    <tr>
        <th> USER </th>
        <th> SUBJECT </th>
    </tr>

<?php
$result = $conn->query($sql); 
$sql = "SELECT * FROM users WHERE rol='profesor'";
$result = $conn->query($sql);

$sql2 = "SELECT * FROM users_subjects WHERE id=$id";
$result2 = $conn->query($sql2);

if ($result->num_rows==0){

    echo 'No professors >';
}else{
    while($row = $result->fetch_assoc()) {
        echo "<tr>
                  <td>".$row["username"]."</td>
                  <td>".$row["subject"]."</td>
                  </tr>";
    }
}

?>
</table>

Answer 1

Change the $sql2 query to :-

$sql2 = "SELECT s.subject_name,u.username FROM users_subjects us LEFT JOIN users ON us.user_id=u.id 
LEFT JOIN subjects ON s.subject_id = us.subject_id  WHERE users_subjects_id=$id";

And replace the while loop with the following :- 
while($row = $result->fetch_assoc()) {
        $subjects = implode(',',$row['subject_name']);
        echo "<tr>
                  <td>".$row["username"]."</td>
                  <td>".$subjects."</td>
                  </tr>";
 }

You can take a look but do something like this.

Thanks