Database Problem: “The page isnt working, if the problem continues, contact the site owner.” [closed]

I created a database in phpmyadmin,
I wrote PHP code in connect.php,
I wrote code in signin.html,
I connected them all together.
But still, it does not work.

Here is my signin.html code:

<form method="post" action="connect.php">
<label>First Name...</label>
<input type="text" placeholder="" id="firstName" name="firstName">
<label>Last Name...</label>
<input type="text" placeholder="" id="lastName" name="lastName">
<label>Email</label>
<input type="email" placeholder="" id="email" name="email">
<label>Password...</label>
<input type="password" placeholder="" id="password" name="password">
<label>Comfirm Password...</label>
<input type="password" placeholder="">
<input type="submit" value="Submit" name="btn btn-primary">
</form>

---

Here is my connect.php code:

    <?php
$firstName = filter_input(INPUT_POST, 'firstName');
$lastName = filter_input(INPUT_POST, 'lastName');
$email = filter_input(INPUT_POST, 'email');
$password = filter_input(INPUT_POST, 'password');
if (!empty($firstName)){
if (!empty($lastName)){
if (!empty($email)){
if (!empty($password)){
$host = "localhost";
$dbusername = "root"; 



$dbpassword = "";
$dbname = "accounts";
// Create connection
$conn = new mysqli ($host, $dbusername, $dbpassword, $dbname);
if (mysqli_connect_error()){
die('Connect Error ('. mysqli_connect_error() .') '
. mysqli_connect_error());
}
else{
$sql = "INSERT INTO registration (firstName, lastName, email, password)
values ('$firstName', '$lastName', '$email', '$password')";
if ($conn->query($sql))}
echo "New record is inserted sucessfully";
}
else{
echo "Error: ".$sql . "<br>". $conn->error;
}
$conn->close();
}
}
else{
echo "Password should not be empty";
die();
}
}
else{
echo "Username should not be empty"
die();
}
?>

---

I have watched countless of videos of this problem,
but nothing seems to explain the problem.
I know really little about PHP and databases in general.

Thank you for reading through,
I hope you can help.
If you need more information, please contact me.