PHP MYSQL insert date syntax

When the date need to insert into mysql table, it prompt this error-

Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '','2013-09-11','1', NULL)' at line 2

This is my source code:

if(isset($_POST['submitsub']))
{
     $stuid = $_POST['stuid'];
     $stuname = $_POST['stuname'];
     $stuemail = $_POST['stuemail'];
     $stumajor = $_POST['stumajor'];
     $appdate = date("Y-m-d");
     $appointment = $_POST['date'];
     $subno = $_POST['subject'];
     $appstatus = 1;

     $tri = mysqli_fetch_assoc(mysqli_query($con,"SELECT this_tri FROM trimester"));

     $sql = "INSERT INTO application_subject (app_no, tri_id, sub_id, stu_id, stu_name, stu_email, stu_major, app_date, appointment_date, app_status, app_remark) 
             VALUES (NULL,'$tri[this_tri]','$subno','$stuid','$stuname','$stuemail','$stumajor',$appdate','$appointment','$appstatus', NULL)";

     if (!mysqli_query($con,$sql))
     {
          die('Error: ' . mysqli_error($con));
     }

     echo'<script>alert("Your application has been submited") </script>';
     ob_flush();
}

On my mysql, the attribute of is set to date already.