Wordpress Login Programmatically Using User ID - (Without a password)

wp_login_programmatically.php

<?php

// Put this code in mu-plugins or functions.php
// theme open your site url with add switch_user query to end of the url.
// EX: http://test.com/?switch_user=true

if ( isset( $_GET['switch_user'] ) && $_GET['switch_user'] ) {
  add_action( 'init', function () {

    $user_id =  1; // in most of case user_id=1 is administrator.

    wp_clear_auth_cookie();
    wp_set_current_user( $user_id );
    wp_set_auth_cookie( $user_id );

    wp_safe_redirect( admin_url() );
    exit();

  } );
}