HttpURLConnection to use connect to php Error:Server returned HTTP response code: 500 for URL:

I'm using java to read from an excel file and put that data into variables, then send those variables to php where it will get processed and inserted into a mysql database. The database wont allow me to directly insert into the database, so I'm doing a work-around. I get this error when I run my program. "Server returned HTTP response code: 500 for URL:" This happens when it gets to the conn.getOutputStream(); If someone knows how i can fix this on the server side please let me know so that I can try to fix it

String link="http://website/directory/file.php";
            String data  = URLEncoder.encode("fName", "UTF-8")
                    + "=" + URLEncoder.encode(fName, "UTF-8");
            data += "&" + URLEncoder.encode("lName", "UTF-8")
                    + "=" + URLEncoder.encode(lName, "UTF-8");
            data += "&" + URLEncoder.encode("SID", "UTF-8")
                    + "=" + URLEncoder.encode(SID+"", "UTF-8");
            data += "&" + URLEncoder.encode("email", "UTF-8")
                    + "=" + URLEncoder.encode(email, "UTF-8");
            data += "&" + URLEncoder.encode("major", "UTF-8")
                    + "=" + URLEncoder.encode(major, "UTF-8");
            data += "&" + URLEncoder.encode("year", "UTF-8")
                    + "=" + URLEncoder.encode(year, "UTF-8");
            data += "&" + URLEncoder.encode("isPaid", "UTF-8")
                    + "=" + URLEncoder.encode(isPaid+"", "UTF-8");
            data += "&" + URLEncoder.encode("isNational", "UTF-8")
                    + "=" + URLEncoder.encode(isNational+"", "UTF-8");
            URL url = new URL(link);
            //Did the CookieHandler just in case the server wants them
            CookieHandler.setDefault(new CookieManager(null, CookiePolicy.ACCEPT_ALL));

            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setDoOutput(true);

            OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
            wr.write( data );
            System.out.println("data was passed");
            wr.flush();
            BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            StringBuilder sb = new StringBuilder();
            String line = null;

            // Read Server Response I'm using echo . ie: echo "yes";
            while((line = reader.readLine()) != null)
            {
                sb.append(line);
                break;
            }
            System.out.println(sb.toString());
        }
            if(sb.toString().equals("yes"))
            {
                ttlAffected++;
            }
            else if(sb.toString().equals("no"))
            {
                System.out.println("Student ID: "+array.get(rowOn).getSID()+" already exists");
                fail.add(array.get(rowOn).getSID());
            }
            else if(sb.toString().equals("error"))
            {
                System.out.println("An unknown error occured");
                fail.add(array.get(rowOn).getSID());
            }
            else
            {
                System.out.println("IDk what happened        "+  sb.toString());
            }

Also, here is the php side

    <?php
    $con=mysqli_connect("host","username","password","database");
    if (mysqli_connect_errno($con))
{
   echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$_SESSION['fName']=$_POST['fName'];
$_SESSION['lName']=$_POST['lName'];
$_SESSION['SID']=$_POST['SID'];
$SID=$_SESSION['SID']
$_SESSION['email']=$_POST['email'];
$_SESSION['major']=$_POST['major'];
$_SESSION['year']=$_POST['year'];
$_SESSION['isNational']=$_POST['isNational'];
$_SESSION['isPaid']=$_POST['isPaid'];


$sql ="SELECT * FROM Registered_Members15 WHERE SID='$SID'";
$result = mysqli_query($com,$sql);
if(mysqli_num_rows($result)==0)
{
                                        $query_member = "INSERT INTO Events15(id,SID) VALUES (?, ?)";
                                        $stmt_member = mysqli_prepare($con,$query_member);
                                        mysqli_stmt_bind_param($stmt_member, "si" , $_SESSION['email'], $_SESSION['SID']);
                                        mysqli_stmt_execute($stmt_member);
                                        $affected_rows_mem = mysqli_stmt_affected_rows($stmt_member);
                                        mysqli_stmt_close($stmt_member);

                                        $query = "INSERT INTO Registered_Members15(ID,first_name,last_name,SID,email,paid_chapter,paid_national,major,year)  VALUES (NULL,?, ?, ?, ?, ?, ?, ?, ?)";
                                        $stmt = mysqli_prepare($con, $query);
                                        mysqli_stmt_bind_param($stmt, "ssisiiss", $_SESSION['fName'], $_SESSION['lName'], $_SESSION['SID'], $_SESSION['email'], $_SESSION['isPaid'], $_SESSION['isNational'], $_SESSION['major'], $_SESSION['year']);
                                        mysqli_stmt_execute($stmt);

                                         $affected_rows = mysqli_stmt_affected_rows($stmt);
                                        if($affected_rows == 1 && $affected_rows_mem == 1)
                                        {
                                                echo "yes";
                                        }
                                        else
                                        {
                                            echo "error";
                                        }
}else{
    echo "no";
}

mysqli_close($con);

?>

I believe this is all right but I do want to ask if someone could check if i did something wrong.

via Chebli Mohamed