Сначала отправляете юзера на этот url
$params = [
'client_id' => 'ваш app_id',
'redirect_uri' => 'http://site.tld/vk.php',
'display' => 'page',
'scope' => 'email',
'response_type' => 'code',
'v' => '5.60',
];
$url = 'https://oauth.vk.com/authorize?' . http_build_query($params, null, '&');
скрипт vk.php
$params = [
'client_id' => 'ваш app_id',
'client_secret' => 'ваш app secret',
'redirect_uri' => 'http://site.tld/vk.php',
'code' => $_GET['code'],
];
$token = file_get_contents('https://oauth.vk.com/access_token?' . http_build_query($params, null, '&'));
//ответ будет такой {"access_token":"****","expires_in":"*****","user_id":"*****","email":"****"}