Stuck trying to create a login using object-orientation

Question

I am following this tutorial to create a login and register using OO PHP.

https://www.youtube.com/watch?v=AtivJV-kx5c&list=PLfdtiltiRHWF5Rhuk7k4UAU1_yLAZzhWc#t=364

When I enter in the correct username and password it is just displaying a blank screen. So far I have been able to follow the videos with only minor issues which I have been able to fix.

This is the code I have for the login page, can anyone see something obviously wrong with it?

<?php require_once 'core/init.php';

if(input::exists()) {
 
 $validate = new validate();
 $validation = $validate->check($_POST, array(
   'username'=> array('required' => true),
   'password'=> array('required' => true)

  ));

  if($validation->passed()) {
   $user = new user();
   $login = $user->login(input::get('username'), input::get('password'));

   if($login) {
    echo 'success';
   } else {
    echo 'loggin failed';
   }

  } else {
   foreach($validation->errors() as $error) {
    echo $error . '<br>';
   }
  }
}

?>


<form action="login.php" method="post">
 
 <div class="field">
  <label for="username">Username:</label>
  <input type="text" name="username" id="username" autocomplete="off">
 </div>

 <div class="field">
  <label for="password">Password:</label>
  <input type="password" name="password" id="password">
 </div>

 <input type="submit" value="Log in">
</form>

I'm just trying to get it to display the word 'success' at the moment when I use the wrong log in details. As it is at the moment it just displays a blank page if i put in the wrong log in details, but if i leave it black it will give me the error I had wrote.

---- EDIT ----

some more code that are on different files to help you make a bit more sense of what I'm doing...

<?php

class input {
 public static function exists($type = 'post') {
  switch($type) {
   case 'post':
    return (!empty($_POST)) ? true : false;
   break;
   case 'get':
    return (!empty($_GET)) ? true : false;
   break;
   defult:
    return false;
   break;
  }
 }

 public static function get($item) {
  if(isset($_POST[$item])) {  //check for post data
   return $_POST[$item];  //if available return data
  } else if(isset($_GET[$item])) {  
   return $_GET[$item];  //if not return get data
  }
  return ''; 
 }
}

?>

and...

<?php 
class user {
 private $_db,
   $_data;

 public function __construct($user = null) {
  $this->_db = DB::getInstance();
 }

 public function create($fields = array()) {
  if(!$this->_db->insert('users', $fields)) {
   throw new Exception('There was a problem when creating the account.');
  }
 }

 public function find($user = null) {  //find a user by their ID
  if($user) {
   $field = (is_numeric($user)) ? 'id' : 'username';
   $data = $this->_db->get('users', array($field, '=', $user));

   if($data->count()) {
    $this->_data = $data->first();
    return true;
   }
  }
  return false;
 }

 public function login($username = null, $password = null) {
  $user = $this->find($username);
  print_r($user);

  return false;
 }


}

?>

and...

<?php
class validate {
 private $_passed = false,
   $_errors = array(),
   $_db = null;

 public function __construct() {
  $this->_db = DB::getInstance();
 }

 public function check($source, $items = array()) {
  foreach($items as $item => $rules) {
   foreach($rules as $rule => $rule_value) {

    $value = trim($source[$item]);
    $item = escape($item);

    if($rule === 'required' && empty($value)) {
     $this->addError("{$item} is required");
    } else if(!empty($value)) {
     switch($rule) {
      case 'min':
       if(strlen($value) < $rule_value) {
        $this->addError("{$item} must be a minimum of {$rule_value} characters.");
       }

      break;
      case 'max':
       if(strlen($value) > $rule_value) {
        $this->addError("{$item} must not be more than {$rule_value} characters.");
       }

      break;
      case 'matches':
       if($value != $source[$rule_value]) {
        $this->addError("{$item} must match {$rule_value}.");
       }

      break;
      case 'unique':
       $check = $this->_db->get($rule_value, array($item, '=', $value));
       if($check->count()) {
        $this->addError("{$item} already exists. Please enter a different Username.");
       }

      break;
     }
    }
   }

  }

   if(empty($this->_errors)) {
    $this->_passed = true;
   }

   return $this;
 }

 private function addError($error) {
  $this->_errors[] = $error;
 }

 public function errors() {
  return $this->_errors;
 }

 public function passed() {
  return $this->_passed;
 }
}