<?php include "../connect.php"; ?>
<?php
  if(isset($_POST['submit'])) {
    global $username;
    global $password;
    $username = $_POST['username'];
    $password = $_POST['password'];
    if($username && $password) {
      connect();
  }
}
?>


// а вот и сама функция


<?php include "../login.php"; ?>
<?php
  function connect() {
    global $connection;
    $connection = mysqli_connect('localhost', 'root', 'root', 'loginapp');
    $query = "INSERT INTO users(username, password) ";
    $query .= "VALUES('$username', '$password')";
    $result = mysqli_query($connection, $query);
  }
?>