Database is empty - php

Question

My database is empty. I don't know what's wrong any more. Is it a syntax error? I've already tried to find a syntax error, but I can't find one. Can anyone help me or tell me what's the problem?
<?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]*%/", $username)) {
header("Location: ../signup.php?error=invalidmail&uid=");
exit();
}
elseif (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
header("Location: ../signup.php?
error=invalidmail&uid=".$username);
exit();
}
elseif (!preg_match("/^[a-zA-Z0-9]*$/", $username)) {
header("Location: ../signup.php?error=invalidmail&uid=".$email);
exit();
}
elseif ($password !== $passwordRepeat) {
header("Location: ../signup.php?
error=passworcheck&uid=".$username."&mail=".$email);
}
else {
$sql = "SELECT uidUsers FROM users WHERE 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, emailUsers, 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, "ssss", $username, $email,
$password);
mysqli_stmt_execute($stmt);
header("Location: ../signup.php?signup=succes");
exit();
}
}
}
}
mysqli_stmt_close($stmt);
mysqli_close($conn);
}
else {
header("Location: ../signup.php");
exit();
}

Answer 1

My guess: This line is wrong:
$sql = "SELECT uidUsers FROM users WHERE uidUsers=?";
^^^^^^^^^
....
mysqli_stmt_bind_param($stmt, "s", $username);
I think, you want to ask fo a user name to get the user id.
There maybe more errors.