Login system PHP SQL error

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PHP

Hi,

I'm making a PHP login system through a tutorial, but I got an error I don't know how to solve it..
In my url i get a sqlerror.
I got a dbhandler in another file but i'm sure that is not the issue because it's working in another file.
I checked for syntax errors and that is not the issue..
Can somebody help me?
This is my code:

<pre><?php
if (isset($_POST['signup-submit'])) {

  require 'dbh.inc.php';

  $username = $_POST['uid'];
  $email = $_POST['mail'];
  $password = $_POST['pwd'];
  $passwordrepeat = $_POST['pwd-repeat'];

if (empty($username) || empty($email) || empty($password) || empty($passwordrepeat)) {
    header("Location: ../signup.php?error=emptyfields&uid=".$username."&mail=".$email);
    exit();
} elseif (!filter_var($email, FILTER_VALIDATE_EMAIL) && !preg_match("/^[a-zA-Z0-9]*$/", $username)) {
    header("Location: ../signup.php?error=invalidmailuid=");
    exit();
} elseif (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
    header("Location: ../signup.php?error=invalidmail&uid=".$username);
    exit();
} elseif (!preg_match("/^[a-zA-Z0-9]*$/", $username)) {
    header("Location: ../signup.php?error=invaliduid&mail=".$email);
    exit();
} elseif ($password !== $passwordrepeat) {
    header("Location: ../signup.php?error=passwordcheck&uid=".$username."&mail=".$email);
    exit();
} else {

  $sql = "SELECT uidUsers FROM users uidUsers=?";
  $stmt = mysqli_stmt_init($conn);
  if (!mysqli_stmt_prepare($stmt, $sql)) {
    header("Location: ../signup.php?error=sqlerror");
    exit();
} else {
  mysqli_stmt_bind_param($stmt, "s", $username);
  mysqli_stmt_execute($stmt);
  mysqli_stmt_store_result($stmt);
  $resultCheck = mysqli_stmt_num_rows($stmt);
  if ($resultCheck > 0) {
    header("Location: ../signup.php?error=usertaken&mail=".$email);
    exit();
} else {

  $sql = "INSERT INTO users (uidUsers, mailusers, pwdUsers) VALUES (?, ?, ?)";
  $stmt = mysqli_stmt_init($conn);
  if (!mysqli_stmt_prepare($stmt, $sql)) {
    header("Location: ../signup.php?error=sqlerror");
    exit();
} else {
  $hashedPwd = password_hash($password, PASSWORD_DEFAULT);

  mysqli_stmt_bind_param($stmt, "sss", $username, $email, $hashedPwd);
  mysqli_stmt_execute($stmt);
  header("Location: ../signup.php?signup=success");
  exit();
}

}

}

}
 mysqli_stmt_close($stmt);
 mysqli_close($conn);

} else {
  header("Location: ../signup.php");
  exit();
}

Thanks Sam

What I have tried:

I tried to follow the tutorial for a simple login system from mmtuts on yt.