Login programmatically

login.php

<?php

/**
 * Drupal 7.
 */ 
$user_obj = user_load(1);
$form_state = array();
$form_state['uid'] = $user_obj->uid;
user_login_submit(array(), $form_state);

/**
 * Drupal 6.
 */ 
$user_obj = user_load(170);

$state = array();
$state['values']['uid'] = $user_obj->uid;

$state['values']['name'] = $user_obj->name;

// Valid login.
global $user;
$user = $user_obj;
user_authenticate_finalize($state['values']);