HTML Form doesn't send data to the database - php

Question

I know there are questions similar to this but their answers doesn't solve my problem.
My php is running without errors (at least not from the navigator), it even returns me the custom url I set at the end of my script ?register=success but it doesn't send the data to my database(local).
I really don't know what section of my code I should let here because, as I said at the begining, there are not errors from php in the navigator, so I just put all my main php (register.php).
<?php
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"]);
$nickname = mysqli_real_escape_string($conn, $_POST["nickname"]);
$password = mysqli_real_escape_string($conn, $_POST["password"]);
$email = mysqli_real_escape_string($conn, $_POST["email"]);
//Birthdate
$birthDay = mysqli_real_escape_string($conn, $_POST["birth-day"]);
$birthMonth = mysqli_real_escape_string($conn, $_POST["birth-month"]);
$birthAge = mysqli_real_escape_string($conn, $_POST["birth-age"]);
$birthDate = $birthDay."-".$birthMonth."-".$birthAge;
$speciality = mysqli_real_escape_string($conn, $_POST["speciality"]);
$gender = mysqli_real_escape_string($conn, $_POST["gender"]);
//Error handlers
//Check for empty fields
if (empty($first) || empty($last) || empty($email) || empty($last) || empty($last) || empty($last)) {
header("Location: ../register.php?register=empty");
exit();
}else{
//Check if input characters are valid
if(!preg_match("/^[a-zA-Z]*$/", $first) || !preg_match("/^[a-zA-Z]*$/", $last)){
header("Location: ../register.php?register=invalid");
exit();
}else{
//Check if email is valid
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
header("Location: ../register.php?register=email");
exit();
}else{
//Check if birthdate is valid
if (!checkdate ($birthMonth , $birthDay, $birthAge)) {
header("Location: ../register.php?register=date");
exit();
}else{
//Check if user exists
$sql = "SELECT * FROM users WHERE user_nickname='$nickname'";
$result = mysqli_query($conn, $sql);
$resultCheck = mysqli_num_rows($result);
//Entering to the database to check if username exists
if ($resultCheck > 0) {
header("Location: ../register.php?register=usertaked");
exit();
}else{
//Hashing the password
$hashedPwd = password_hash($password, PASSWORD_DEFAULT);
//Insert user registration data into the database
$sql = "INSERT INTO users (user_first, user_last, user_nickname, user_password, user_email, user_birth_date, user_speciality, user_gender) VALUES ('$first', '$last', '$nickname', '$password', '$email', '$birthDate', '$speciality', '$gender')";
mysqli_query($conn, $sql);
header("Location: ../register.php?register=success");
exit();
}
}
}
}
}
}else{
header("Location: ../register.php");
exit();
}
And this is the php where I do the connection with the database (dbh_inc.php):
<?php
$dbServername = "localhost";//because there's running a local server
$dbUsername = "root";
$dbPassword = "";
$dbName = "login_system";
$conn = mysqli_connect($dbServername, $dbUsername, $dbPassword, $dbName) or die ("Conexion fallida");