SQL statement in php will not insert onto database

Question

I have this SQL statement that i'm trying to get to save new students to a table of students, however it simply isn't doing it, I don't get any error messages when I run error reporting and I ran the Query in sqlbuddy with values swapped in and it worked fine. Any ideas on what im doing wrong will be appreciated.

Heres the code:

<?php
session_start();
ini_set('display_startup_errors',1);
ini_set('display_errors',1);
error_reporting(-1);
$default = 'default';
$ClassID = $_GET['ID'];
$Surname = $_POST['Surname'];
$Firstname = $_POST['Firstname'];
$Firstletter = $Firstname[0];
$Username = $Firstletter + $Surname;
$sql_link = mysqli_connect('localhost', 'root', 'password', 'GameData');
$counter = mysqli_query($sql_link,"SELECT * FROM IDCounter");
$counter = mysqli_fetch_array($counter);
mysqli_query($sql_link,"INSERT INTO tblStudents(StudentID, StudentFirstName,    StudentSurname, ClassID, UserName, Password, CharacterSelect)
VALUES ('$counter[Counter]', '$_POST[Firstname]', '$_POST[Surname]', '$ClassID',  '$Username', '$default', 1)");
mysqli_close($sql_link);
header ("Location: TeacherSide.php");
?>

The POST values come from the form that directs to this page

Answer 1

我刚刚解决了我遇到的问题，很遗憾地告诉您，这是一个非常愚蠢的问题，我没有更新计数器，因此，每次尝试添加新学生时，它都会尝试使用相同的StudentID，因此失败，一个简单的解决方法