No database selected - php and phpmyadmin - php

Question

I am trying to connect my php code to my database and once I input and submit things like username, email address, and password in my html form. They will be sent to my database and shown in specific columns and there will be text like "New record has been created successfully" on my page. But for now, an error occurs instead.
The following is my php code
<?php
$visitorname = $emailaddress = $visitorpassword = $confirmpassword = "";
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "mydb";
if($_SERVER["REQUEST_METHOD"] == "POST"){
$visitorname = input($_POST["visitorname"]);
$emailaddress = input($_POST["emailaddress"]);
$visitorpassword = input($_POST["visitorpassword"]);
$confirmpassword = input($_POST["confirmpassword"]);
$hashed_password = password_hash($password, PASSWORD_DEFAULT);
}
function input($data){
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
if($visitorpassword == $confirmpassword){
// Create connection
$conn = new mysqli($servername, $dbname);
// Check connection
if ($conn->connect_error) {
die("Connection failed: " . $conn->connect_error);
}
echo "Connected successfully";
$id = mysqli_insert_id($conn);
$sql = "INSERT INTO registereduser (id, username, emailaddress,
hashedpassword) VALUES ($id, $visitorname, $emailaddress,
$hashed_password)";
if ($conn->query($sql) === TRUE){
echo "New record created successfully";
}else{
echo "Errors " . $sql . "<br>" . $conn->error;
}
$conn->close();
}else{
echo "Passwords do not match";
}
?>
The following is the error output shown on my web page:
Connected successfullyErrors INSERT INTO registereduser (id, username, emailaddress, hashedpassword) VALUES (0, Josh5577, josh1998#hotmail.com, $2y$10$RMasEdVOskmcXbmfchLeBeUzLa5l38jXFaCQN7vMEhR8A0mU/iiC6)
No database selected

Answer 1

Please use below code for connection
$conn = new mysqli($servername, $username, $password, $dbname);

Answer 2

You can try to change
// Create connection
$conn = new mysqli($servername, $dbname);
to
// Create connection
$conn = new mysqli($servername, $username, $password);
mysqli_select_db($conn, $dbname);