// вот 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);
      }
    ?>




    // а вот основной файл

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