OK.

Firstly the correct version fo the code you posted above is

Code:
<form action="./write.php" method="post">
  <input type="checkbox" name="id" value="1" />
  <input type="checkbox" name="id" value="2" />
  <input type="checkbox" name="id" value="3" />

  <input type="submit" value="Add this/these Songs to my PlayList" name="B1" />
</form>
Now I suggest you read this, it should provide you with the inslight you need

Cheers,

Ryan Jones