3 comments

1. The Result

   

   Username

   You can declare the form field value global and prefill it.

   This will override any setting done via the “Remember me”-Checkbox.

   /**
    * Changes the default user name to "DEMO"
    * 
    * @return string $user_login
    */
   function login_form_username()
   {
       global $user_login;
       return $user_login = 'DEMO';
   }
   add_action( 'login_head', 'login_form_username' );
   

   Password

   I found nothing that you can use to prefill it, so I’d add a note. Gladly/Sadly there’s the whole set of stylesheets for the admin UI available. So .wrap in connection with h2 gives a nice styling. The function is hooked to the additional fields hook for the admin hook. The priority is 0 to set it above additional fields.

   /**
    * Adds a note beyond the user login name & password field
    * 
    * @return string
    */
   function login_form_note()
   {
       print '<div class="wrap"><h2 style="text-align: center;">The Password is "DEMO"</h2></div>';
   }
   add_action( 'login_form', 'login_form_note', 0 );
   

   Security: Hide Errors

   As someone would get a visual proof that he entered an existing user name by default, I’d add the following to your functions.php file, to avoid telling if the username was guessed right:

   /**
    * Hide wrong login names
    * 
    * @return string
    */
   function no_login_error() 
   {
       return __( 'Wrong Credentials.' );
   }
   add_filter( 'login_errors', 'no_login_error' );
   

   Other filters

   Notes:

   • If you want to replace other stuff like styles, then you can use the login_enqueue_scripts hook.
   • You can also replace the link behind the logo using the login_headerurl filter that filters the url. The link title can be replaced using login_headertitle. Both trigger for multisite as well a single site setups.
   • The login message can be changed using the login_message filter.

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2. you could use this code to auto login the user:

   add_action('init', 'auto_login');
   add_action('admin_init', 'auto_login');
   function auto_login() {
       if (!is_user_logged_in()) {
         //by user name
         $user = get_user_by( 'login', 'demo' );
         //Or by user id, 2 being the ID of the demo user
         //$user = get_userdata(2);
         wp_set_current_user($user->ID, $user->user_login);
         wp_set_auth_cookie($user->ID);
         do_action('wp_login', $user->user_login);
       }
   }
   

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3. Here are 3 ways to do it:

   The first way is to use the wp_signon hook

   $creds = array();    
   $creds['user_login'] = 'example';   
   $creds['user_password'] = 'plaintextpw';   
   $creds['remember'] = true;   
   $user = wp_signon( $creds, false );  
   if ( is_wp_error($user) ) {  
      echo $user->get_error_message();
   }
   

   The second way, is to use jQuery .val() and add it to your login page

   $("#user_login").val('username');    
   $("#user_pass").val('password');
   

   The last way is to edit your wp-login.php file (Note, this is not recommended although it would work).

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