无法插入数据库但未显示错误

Question

I have a problem with my PHP code. The insert into.. query does not work, but no connection error is shown. It seems like the function is never called. What am I doing wrong?

function addGroup(){
    require_once('config.php');
    $gid=mysql_real_escape_string($_POST['gid']);
    $sql_add_group="INSERT INTO groups(gid) VALUES(".$gid.");";
    $result = mysql_query($sql_add_group, $link);

    if (! $result) {
       die('Could not get data: ' . mysql_error());
    }

    mysql_close($link);
}

if(isset($_POST['Add Group'])){
    addGroup();
}

if($_SESSION['type']=='professor'){   
    echo $_POST['gid'];
    echo('<div>
            <form action="groups.php" method="POST">
              <label>Add/Remove Group<input class = "form-control" type = "text" name = "gid" id = "gid" placeholder = "group#"/></label>
              <input id="button" type="submit" value="Add Group" name="Add Group"> </input>
              <input id="button" type="submit" value="Remove Group" name="Remove Group"> </input>
            </form>
          </div><br/>');
}