Signup page php

<?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();
}
else if (!filter_var($email, FILTER_VALIDATE_EMAIL) && !preg_match("/^[a-zA-Z0-9]*$/", $username)) {
header("Location: ../signup.php?error=invaliduid");
exit();
}
else if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
 header("Location: ../signup.php?error=invalidmail&uid=".$username);
exit();
}
else if (!preg_match("/^[a-zA-Z0-9]*$/", $username)) {
 header("Location: ../signup.php?error=invaliduid&mail=".$email);
exit();
}
 else if ($password !== $passwordRepeat) {
 header("Location: ../signup.php?error=passwordcheck&uid=".$username."&mail=".$email);
 exit();
}
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, "sss", $username, $email, $hashedPwd);
 mysqli_stmt_execute($stmt);
 header("Location: ../signup.php?signup=success");
exit();

}

}

}

}
}

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

?>