MySQL & PHP Sign-up Form - php

Question

Hy :) Basically i am struggling with the same Problem as this Boy over here: PHP Sign-up Form Not Working - I think we were going through the same youtube tutorial :D
unfortunately i could not comment on this topic, that is why i am opening a new one. The thing is that my signup.php should be correct (at least the inputs right?) , but i am very likely to overlook things:
Thi signup.php File:
<?php
include_once 'header.php';
?>
<section class="main-container">
<div class="main-wrapper">
<h2>Sign up</h2>
<form class="signup-form" action="includes/signup.inc.php" method="POST">
<input type="text" name="first" placeholder="Firstname">
<input type="text" name="last" placeholder="Lastname">
<input type="text" name="email" placeholder="E-Mail">
<input type="text" name="uid" placeholder="Username">
<input type="password" name="pwd" placeholder="Password">
<button type="submit" name="submit">Sign up</button>
</form>
</div>
</section>
<?php
include_once 'footer.php';
?>
When I type in information, it keeps on displaying me signup.php?signup=empty.
if (isset($_POST['submit'])) {
include_once 'dbh.inc.php';
$first = mysqli_real_escape_string($conn, $_POST['first']);
$last = mysqli_real_escape_string($conn, $_POST['last']);
$email = mysqli_real_escape_string($conn, $_POST['email']);
$uid = mysqli_real_escape_string($conn, $_POST['uid']);
$pwd = mysqli_real_escape_string($conn, $_POST['pwd']);
//Error Handlers
//Check for empty fieldset
if (empty($first) || empty($last) || empty($email) || empty($uid) || empty($pwd)) {
header("Location: ../signup.php?signup=empty");
exit();
}
else {
//Check if input characters are valid
if (!preg_match("/^[a-zA-Z]*$/", $first) || !preg_match("/^[a-zA-Z]*$/", $last)) {
header("Location: ../signup.php?signup=invalid");
exit();
}
else {
// Check if E-Mail is valid
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
header("Location: ../signup.php?signup=email");
exit();
}
else {
$sql = "SELECT * FROM users WHERE user_uid='$uid'";
$result = mysqli_query($conn, $sql);
$resultCheck = mysqli_num_rows($result);
if ($resultCheck > 0) {
header("Location: ../signup.php?signup=usertaken");
exit();
}
else {
// Hashing the Password (verschlüsseln)...
$hashedPwd = password_hash($pwd, PASSWORD_DEFAULT);
//Insert the User into the Database
$sql = "INSERT INTO users (user_first, user_last, user_email, user_uid, user_pwd) VALUES ('$first', '$last', '$email', '$uid', '$hashedPwd');";
mysqli_query($conn, $sql);
header("Location: ../signup.php?signup=success");
exit();
}
}
}
}
}
else {
header("Location: ../signup.php");
exit();
}
Thank you very much in advance for any hint on this.
Best, Chris
update:
When adding following to the Code:
exit(var_dump(empty($first), empty($last), empty($email), empty($uid), empty($pwd)));
it returns
bool(true) bool(true) bool(true) bool(true) bool(true)