Can I programmatically login a user without a password?

I’m manually creating users programmatically, and I want to sign in the newly created user. WP makes it easy to access the hashed password, but not the plaintext version. Is there a way to use wp_signon() without the plaintext password?

I found one person who claims to have done this here, but it didn’t work for me.

THANKS!

Solutions Collecting From Web of "Can I programmatically login a user without a password?"

wp_set_auth_cookie() will log a user in without having to know their password.

The following code does the job for automatic login, without any password!

// Automatic login //
$username = "Admin";
$user = get_user_by('login', $username );

// Redirect URL //
if ( !is_wp_error( $user ) )
{
    wp_clear_auth_cookie();
    wp_set_current_user ( $user->ID );
    wp_set_auth_cookie  ( $user->ID );

    $redirect_to = user_admin_url();
    wp_safe_redirect( $redirect_to );
    exit();
}

I have found another solution here that uses a better approach (at least in my opinion…).
No need to set any cookie, it uses the WordPress API:

/**
 * Programmatically logs a user in
 * 
 * @param string $username
 * @return bool True if the login was successful; false if it wasn't
 */
    function programmatic_login( $username ) {
        if ( is_user_logged_in() ) {
            wp_logout();
        }

    add_filter( 'authenticate', 'allow_programmatic_login', 10, 3 );    // hook in earlier than other callbacks to short-circuit them
    $user = wp_signon( array( 'user_login' => $username ) );
    remove_filter( 'authenticate', 'allow_programmatic_login', 10, 3 );

    if ( is_a( $user, 'WP_User' ) ) {
        wp_set_current_user( $user->ID, $user->user_login );

        if ( is_user_logged_in() ) {
            return true;
        }
    }

    return false;
 }

 /**
  * An 'authenticate' filter callback that authenticates the user using only     the username.
  *
  * To avoid potential security vulnerabilities, this should only be used in     the context of a programmatic login,
  * and unhooked immediately after it fires.
  * 
  * @param WP_User $user
  * @param string $username
  * @param string $password
  * @return bool|WP_User a WP_User object if the username matched an existing user, or false if it didn't
  */
 function allow_programmatic_login( $user, $username, $password ) {
    return get_user_by( 'login', $username );
 }

I think the code is self explanatory:

The filter searches for the WP_User object for the given username and returns it.
A call to the function wp_set_current_user with the WP_User object returned by wp_signon, a check with the function is_user_logged_in to make sure your are logged in, and that’s it!

A nice and clean piece of code in my opinion!

This works well for me:

  clean_user_cache($user->ID);
  wp_clear_auth_cookie();
  wp_set_current_user($user->ID);
  wp_set_auth_cookie($user->ID, true, false);
  update_user_caches($user);

In addition to Mike, Paul and Sjoerd:

To better handle login.php redirections:

//---------------------Automatic login--------------------

if(!is_user_logged_in()){

    $username = "user1";

    if($user=get_user_by('login',$username)){

        clean_user_cache($user->ID);

        wp_clear_auth_cookie();
        wp_set_current_user( $user->ID );
        wp_set_auth_cookie( $user->ID , true, false);

        update_user_caches($user);

        if(is_user_logged_in()){

            $redirect_to = user_admin_url();
            wp_safe_redirect( $redirect_to );
            exit;
        }
    }
}
elseif('http://' . $_SERVER['HTTP_HOST'] . $_SERVER['SCRIPT_NAME'] == wp_login_url()){

    $redirect_to = user_admin_url();
    wp_safe_redirect( $redirect_to );
    exit;
}

To be placed in wp-config.php just after

require_once(ABSPATH . 'wp-settings.php');

FYI

Based on the above solution, I have released a plugin to keep the user logged in from one wordpress to another by synchronizing user data and cookie session:

https://wordpress.org/plugins/user-session-synchronizer/