Two PHP mysql errors

I am getting these two errors in this PHP file and I do not know how to fix them.

Object of class mysqli_result could not be converted to int in /afs/http://ift.tt/2oPdOPr on line 73

Catchable fatal error: Object of class mysqli_result could not be converted to string in /afs/http://ift.tt/2oPdOPr on line 84

 <?php
    include('config.php');
    ini_set('display_errors', 1);
    ini_set('display_startup_errors', 1);
   error_reporting(E_ALL);
   session_start();

   $user_check = $_SESSION['login_user'];

   $ses_sql = mysqli_query($db,"SELECT username from Student_Record where username = '$user_check'");

    $row = mysqli_fetch_array($ses_sql,MYSQLI_ASSOC);

   $login_session = $row['username'];

   if(!isset($_SESSION['login_user'])){
      header("location:login.php");
   }

   if (isset($_POST['task'])) {
    $selectOption = $_POST['task'];
    }


   switch($selectOption)
    {
    case 2:
    {
      if (isset($_POST['name'])) {
        $name = $_POST['name'];
      }
      $sql = "SELECT * from Student_Record where fullname = '$name'";
      $result=$db->query($sql);
      if ($result->num_rows > 0) {
        $query = "SELECT grades from Student_Record where fullname = '$name'";
        $result1=$db->query($query);
        echo "<br><br><table>";
        while($row = $result1->fetch_assoc()){
        echo "<tr><td>" . $row['grades'] . "</td></tr>";
        }
        echo "</table>";
      }
      else{
        echo "not found";
      }
      break;
      }

    case 1:
    {
      $selection = $_POST['change'];
      $course = $_POST['course'];
      $name = $_POST['name'];
      $sql = "SELECT courses from Student_Record where fullname = '$name'";
      $result = $db->query($sql);

      echo "<h1>Before</h1>";

      if ($result->num_rows > 0) {
            $query = "SELECT courses from Student_Record where fullname = '$name'";
            $result1=$db->query($query);
            echo "<br><br><table>";
            while($row = $result1->fetch_assoc()){
            echo "<tr><td>" . $row['courses'] . "</td></tr>";  
            }
            echo "</table>";
        }
        else{
            echo "not found";
        }

      if($selection="Add"){
        $newcourses = $result + $course;
        $sql = "UPDATE Student_Record SET courses='$newcourses' WHERE fullname='$name'";

        if (mysqli_query($db, $sql)) {
            echo "Record updated successfully";
        } else {
            echo "Error updating record: " . mysqli_error($db);
         }
       }

      if($selection="Drop"){
        $newcourses = str_replace($course,"",$result);
        $sql = "UPDATE Student_Record SET courses='$newcourses' WHERE fullname='$name'";

        if (mysqli_query($db, $sql)) {
            echo "Record updated successfully";
        } else {
            echo "Error updating record: " . mysqli_error($db);
        }
      }

        echo "<h1>Before</h1>";

        if ($result->num_rows > 0) {
            $query = "SELECT * from Student_Record where fullname = '$name'";
                $result1=$db->query($query);
                echo "<br><br><table>";
            while($row = $result1->fetch_assoc()){
            echo "<tr><td>" . $row['fullname'] . "</td><td>" . $row['Student_ID'] . "</td><td>" . $row['email'] . "</td><td>" . $row['courses'] . "</td><td>" . $row['grades'] . "</td></tr>";  
            }
            echo "</table>";
        }
        else{
            echo "not found";
         }
        break;
    }
    default: 
    {
       echo("Error!");
       exit();
       break;
     }
    }

   $checkQuery = "SELECT * from Student_Record WHERE 
   fullname='$_POST[name]'";
  $userCheck = mysqli_query($db, $checkQuery);
  if(!$userCheck){
    echo "Invalid name";
    return false;
    }

   $checkQuery = "SELECT * from Student_Record WHERE 
 Student_ID='$_POST[studentid]'";
   $userCheck = mysqli_query($db, $checkQuery);
  if(!$userCheck){
     echo "Invalid Student ID";
     return false;
   }
 ?>