<form method="post">
<input type="text" name="namedb" value="namedb">
<input type="submit" value="создать запись">
</form>

<?php
if ($_POST){
$servername = "localhost";
$username = "root";
$password = "";

$conn = new mysqli($servername, $username, $password);
if ($conn->connect_error) {
  die("Connection failed: " . $conn->connect_error);
}

$sql = "CREATE DATABASE IF NOT EXISTS Mydatabase";
$conn->query($sql);

$db_selected = $conn->select_db('Mydatabase');

$sql = "CREATE TABLE IF NOT EXISTS MyTable (
id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
login VARCHAR(30)
)";

if ($conn->query($sql) === TRUE) {
  echo "Таблица MyTable успешно создана";
} 

$sql = "INSERT INTO MyTable (login) VALUES ('{$_POST['namedb']}')";

if ($conn->query($sql) === TRUE) {
  echo "<br>Записан следующий логин ".$_POST['namedb'];
}

$sql = "SELECT id,login FROM MyTable";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
  // output data of each row
  while($row = $result->fetch_assoc()) {
    echo "<br>"."id: " . $row["id"]. " Логин: " . $row["login"]."<br>";
  }
} else {
  echo "0 results";
}

$conn->close();

}
?>