Call Magento model (function) from an external file.

call_magento_extern_file.php

<?php
  //include Magento app
  require_once 'app/Mage.php';
  
  //initialize Magento
  Mage::app();
  
  //call Magento model or functions (this example get unsubscription link for customer)
  $email = "test@test.com";
  $unSubLink = Mage::getModel('newsletter/subscriber')->loadByEmail($email)->getUnsubscriptionLink();
  
  echo $unSubLink;  
?>