Print distinct values from a column with php

Question

I want to print in a table some of the subjects from a database table. I have a function that sorts them by class and I have this code fragment to identify, for example, 'math':

<?php foreach ($subjects as $sub): 
   if (stripos($sub['subject'], 'math') !== false): ?>  
      <td><?php echo $sub['subject']; ?></td>
    <?php endif ?>  
<?php endforeach ?>

So, if there is a subject 'math' in this class, it will be printed. But if I have another subject which contains 'math', it continues to print it on next column. Obviously, every subject has a unique id and I guess this can be somehow used to identify every subject, but I cant figure it out how.