Loging in under any customer by using the customer_id only

mage1login.php

<?php

if (version_compare(phpversion(), '5.2.0', '<')) {
    echo 'It looks like you have an invalid PHP version. Magento supports PHP 5.2.0 or newer';
    exit;
}

$magentoRootDir = getcwd();
$bootstrapFilename = $magentoRootDir . '/app/bootstrap.php';
$mageFilename = $magentoRootDir . '/app/Mage.php';

if (!file_exists($bootstrapFilename)) {
    echo 'Bootstrap file not found';
    exit;
}
if (!file_exists($mageFilename)) {
    echo 'Mage file not found';
    exit;
}
require $bootstrapFilename;
require $mageFilename;

if (!Mage::isInstalled()) {
    echo 'Application is not installed yet, please complete install wizard first.';
    exit;
}

//if (isset($_SERVER['MAGE_IS_DEVELOPER_MODE'])) {
Mage::setIsDeveloperMode(true);
//}

ini_set('display_errors', 1);
ini_set('memory_limit', '512M');


umask(0);

/* Store or website code */
$mageRunCode = isset($_SERVER['MAGE_RUN_CODE']) ? $_SERVER['MAGE_RUN_CODE'] : '';

/* Run store or run website */
$mageRunType = isset($_SERVER['MAGE_RUN_TYPE']) ? $_SERVER['MAGE_RUN_TYPE'] : 'store';

Mage::app($mageRunCode, $mageRunType);

if (!isset($_REQUEST['id']) || !is_numeric($_REQUEST['id'])) {
    echo "Usage: {$_SERVER['REQUEST_URI']}?id=%CUSTOMER_ID%";
    die();
}

$customerId = (int)$_REQUEST["id"];
if ($customerId == 0) {
    echo "Error: Customer ID should be Integer value only ";
    exit;
}

try {
    Mage::getSingleton("core/session", array("name" => "frontend"));
    $customer = Mage::getModel("customer/customer")->load($customerId);
    if (!$customer->getId()) {
        echo "Customer ID not found in Magneto : " . $customerId;
        exit;
    }

    /** @var Mage_Customer_Model_Session $session */
    $session = Mage::getSingleton('customer/session');
    $session->setCustomerAsLoggedIn($customer);
    if ($session->isLoggedIn()) {
        echo "You was logged in";
    } else {
        echo "You wasn't logged in";
    }
} catch (Mage_Core_Exception $e) {
    Mage::throwException($e->getMessage());
}

mage2login.php

<?php
namespace mage2login;

use Magento\Customer\Model\ResourceModel\CustomerRepository;
use Magento\Customer\Model\Session;
use Magento\Framework\App\ObjectManager;
use Magento\Framework\App\State;

if (!isset($_REQUEST['id']) || !is_numeric($_REQUEST['id'])) {
    echo "Usage: {$_SERVER['REQUEST_URI']}?id=%CUSTOMER_ID%";
    die();
}

require __DIR__ . '/app/bootstrap.php';
\Magento\Framework\App\Bootstrap::create(BP, $_SERVER);
$om = ObjectManager::getInstance();

$om->get(State::class)->setAreaCode('frontend');
$session = $om->get(Session::class);

$customer = $om->get(CustomerRepository::class)->getById($_REQUEST['id']);

if ($session->loginById($customer->getId())) {
    echo "You was logged in";
} else {
    echo "You wasn't logged in";
}