I am going to describe here some of the methods which can come handy for a theme developer or plugin developer,but are not documented at Wordpress Codex.Today, I am beginning the series with get_userdatabylogin($login_name)

Description

It is one of the important but not documented method on codex(as of now),It provides same information as the get_userdata() except the fact that it takes the login name(e.g. “admin” as an argument rather than the ID of user).It returns an object with the information pertaining to the user whose login name is passed to it. Properties map directly to wp_users table in the database.

Usage

The call to get_userdatabylogin() returns the user’s data, where it can be retrieved using member variables.

<?php $user_info=get_userdatabylogin("admin");
echo ('Username:',$user_info->user_login . '\n');
      echo('User level: ' . $user_info->user_level . '\n');
      echo('User ID: ' . $user_info->ID . '\n');
?>

Username: admin
User level: 10
User ID: 1

Accessing Usermeta Data

<?php $user_info=get_userdatabylogin("admin");
echo ($user_info->last_name .  ", " . $user_info->first_name . "\n");
?>

Singh,Brajesh

Parameters

$login_id

(String) (required) The Login_name(Login Id) of the user whose data should be retrieved.

Default: None

Values in users & user_meta table

Here are the values in the users table you can access via this function:

users

• ID
• user_login
• user_pass
• user_nicename
• user_email
• user_url
• user_registered
• user_activation_key
• user_status
• display_name

useful in user_meta

• first_name
• last_name
• nickname
• user_level
• admin_color (Theme of your admin page. Default is fresh.)
• closedpostboxes_page
• nickname
• primary_blog
• rich_editing
• source_domain
• It works in the same manner as get_userdata() ,except that the argument passed in get_userdata() is an integer(The ID of User),while In the case of get_userdatabylogin() ,the argument passed is string(The Login name e.g. “admin”)

See as a reference :http://codex.wordpress.org/Function_Reference/get_userdata