Option Value "-" gives empty string (HTML PHP) - php

Question

I am trying to POST select values from my HTML form into my Database with phpmysqli.
Somehow the value "-" results as an empty string. The other values are perfectly fine.
So echo $title with value "-" = "" ; with value "Dr." = "Dr."
My HTML:
<select
name="title"
class="form-control"
style="width: 50%;"
required
>
<option value="" disabled selected>Titula</option>
<option value="-">-</option>
<option value="Dr.">Dr.</option>
<option value="Prof. Dr.">Prof. Dr.</option>
</select>
My PHP:
<?php
$username = $_POST['username'];
$password = PASSWORD_HASH($_POST["password"], PASSWORD_DEFAULT);
$title = $_POST['title'];
$mail = $_POST['mail'];
$phone = $_POST['phone'];
$fname = $_POST['fname'];
$lname = $_POST['lname'];
$birth = $_POST['birth'];
$street = $_POST['street'];
$city = $_POST['city'];
$country = $_POST['country'];
$zip = $_POST['zip'];
$father = $_POST['father'];
$mother = $_POST['mother'];
if (!empty($username) || !empty($password) || !empty($birth) || !empty($title) || !empty($mail) || !empty($phone) || !empty($fname) || !empty($lname) || !empty($street) || !empty($city) || !empty($country) || !empty($zip) || !empty($father) || !empty($mother)) {
$host = "localhost";
$dbUsername = "";
$dbPassword = "";
$dbname = "";
//create connection
$conn = new mysqli($host, $dbUsername, $dbPassword, $dbname);
mysqli_set_charset($conn,'utf8mb4');
if (mysqli_connect_error()) {
die('Connect Error('. mysqli_connect_errno().')'. mysqli_connect_error());
} else {
$SELECT = "SELECT mail From users Where mail = ? Limit 1";
$INSERT = "INSERT Into users (username, password, birth, title, mail, phone, fname, lname, street, city, country, zip, father, mother) values(?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)";
//Prepare statement
$stmt = $conn->prepare($SELECT);
$stmt->bind_param("s", $mail);
$stmt->execute();
$stmt->bind_result($mail);
$stmt->store_result();
$rnum = $stmt->num_rows;
if ($rnum==0) {
$stmt->close();
$stmt = $conn->prepare($INSERT);
$stmt->bind_param("ssssssssssssss", $username, $password, $birth, $title, $mail, $phone, $fname, $lname, $street, $city, $country, $zip, $father, $mother);
$stmt->execute();
echo "New record inserted sucessfully";
echo $title; //empty string
//header('Location: login.html');
} else {
echo "Someone already register using this email";
}
$stmt->close();
$conn->close();
}
} else {
echo "All field are required";
die();
}
?>