How do I make a checkbox list with categories, which will update database if it's checked or no?

I got 3 tables:

+----Songs----+   +----Tags----+   +----s_in_tag----+
|     id      |   |     id     |   |       id       |
|    title    |   |    name    |   |     song_id    |
+-------------+   +------------+   |      tag_id    |
                                   +----------------+

In table Songs I store a list of songs, table Tags have a list of tags, and table s_in_tag bind those 2 tables.

Let say i have in Songs:

• id => 1,


• title => song nr1.

In Tags:

• id => 1,


• 
  

  name => POP;

  
  


• 
  

  id => 2,

  
  


• 
  

  name => Rap;

  
  


• 
  

  id => 3,

  
  


• name => Classical.

And in s_in_tag:

• id => 1,


• song_id => 3,


• tag_id => 2

This means that song nr1 has tag Classical

Now the idea is to select a song, to get all records from s_in_tag for that song, and then to output a list of checkboxes with data from tags. I did all this, and i managed to show if a checkbox is selected or no, but how do i make the script which will allow me to add a tag, and then, if i want, to remove him? It does not work with $_POST['checkbox'] because when i remove a tag, i uncheck it.