The $_POST array should not contain the member's id. If the value is stored in the session array, $m_id = $_SESSION['session_member_id']; would be what you want to do.
EDIT: posting in code because screw autoformatting.
@StarLion Basically I have the function that takes $m_id (the actual id of the member logged in and $c_id (as the class id from the input).
I just wanted to pass the $c_id from the input, and take the $m_id from the actual member that is logged in. What you have posted works for what I need, thanks