An Introduction to WP_User Class

WP_User can be found in root/wp-includes/capabilities.php. The constructor takes 3 parameters: $id (int|string::ID|login), $name (string), and $blog_id (int).

You can instantiate a specific user using the class:
[php]<?php
$user = new WP_User( $id [, $name [, $blog_id ] ] );
?>
[/php]

For example:
[php]<?php
$user = new WP_User( 2 );
?>
[/php]

OR, you can get the current user:
[php]<?php
$current_user = wp_get_current_user();
?>
[/php]

While there are normal functions like (e.g., get_the_author_meta() which uses get_userdata() which uses get_user_by() which is new in 3.3.0 and uses WP_User). WP_User has three "magic methods":

1. WP_User::__isset( $key ), which is the same as WP_User::has_prop( $key ).
   Example:
   [php]<?php
   $user = new WP_User( $id );
   if ( $user->__isset( 'myfield' ) )
   // do something
   [/php]
2. WP_User::__get( $key ), which is the same as WP_User::get( $key ).
   Example:
   [php]<?php
   $user = new WP_User( $id );
   if ( $user->__isset( 'myfield' ) )
   $myfield = $user->__get( 'myfield' );
   [/php]
3. WP_User::__set( $key ).
   Example:
   [php]<?php
   $user = new WP_User( $id );
   if ( ! $user->__isset( 'myfield' ) )
   $myfield = $user->__set( 'myfield', $value );
   [/php]

Here are some other methods:

• *Get user data: $user->get_data_by( $field, $value ); // Available fields: 'id', 'slug', 'email' or 'login'
• **Check if user exists: $user->exists();
• *Retrieve the value of a property or meta key: $user->get();
• *Determine whether a property or meta key is set: $user->has_prop();
• Retrieve all the role capabilities merged with the individual capabilities: $user->get_role_caps();
• Add role to a user: $user->add_role( $role );
• Remove role from a user: $user->remove_role( $role );
• Set role for a user: $user->set_role( $role );
• Choose the maximum level the user has: $user->level_reduction( $max, $item );
• Update the maximum user level for the user: $user->update_user_level_from_caps();
• Add cap to a user: $user->add_cap( $cap, $grant );
• Remove cap from a user: $user->remove_cap( $cap );
• Remove all caps from a user: $user->remove_all_caps();
• Determine whether user has a cap: $user->has_cap( $cap, $grant );
• Set blog to operate on: $user->for_blog( $blog_id ); // $blog_id defaults to current blog

*NEW in 3.3
**NEW in 3.4

Here is the object:
[php]
WP_User Object
(
[data] => stdClass Object
(
[ID] => 1
[user_login] => myloginusername
[user_pass] => xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
[user_nicename] => My Nice Name
[user_email] => email@domain.com
[user_url] =>
[user_registered] => 2011-07-15 01:16:18
[user_activation_key] =>
[user_status] => 0
[display_name] => My Display Name
)

[ID] => 1
[caps] => Array
(
[administrator] => 1
)

[cap_key] => wp_capabilities
[roles] => Array
(
[0] => administrator
)

[allcaps] => Array
(
[switch_themes] => 1
[edit_themes] => 1
[activate_plugins] => 1
[edit_plugins] => 1
[edit_users] => 1
[edit_files] => 1
[manage_options] => 1
[moderate_comments] => 1
[manage_categories] => 1
[manage_links] => 1
[upload_files] => 1
[import] => 1
[unfiltered_html] => 1
[edit_posts] => 1
[edit_others_posts] => 1
[edit_published_posts] => 1
[publish_posts] => 1
[edit_pages] => 1
[read] => 1
[level_10] => 1
[level_9] => 1
[level_8] => 1
[level_7] => 1
[level_6] => 1
[level_5] => 1
[level_4] => 1
[level_3] => 1
[level_2] => 1
[level_1] => 1
[level_0] => 1
[edit_others_pages] => 1
[edit_published_pages] => 1
[publish_pages] => 1
[delete_pages] => 1
[delete_others_pages] => 1
[delete_published_pages] => 1
[delete_posts] => 1
[delete_others_posts] => 1
[delete_published_posts] => 1
[delete_private_posts] => 1
[edit_private_posts] => 1
[read_private_posts] => 1
[delete_private_pages] => 1
[edit_private_pages] => 1
[read_private_pages] => 1
[delete_users] => 1
[create_users] => 1
[unfiltered_upload] => 1
[edit_dashboard] => 1
[update_plugins] => 1
[delete_plugins] => 1
[install_plugins] => 1
[update_themes] => 1
[install_themes] => 1
[update_core] => 1
[list_users] => 1
[remove_users] => 1
[add_users] => 1
[promote_users] => 1
[edit_theme_options] => 1
[delete_themes] => 1
[export] => 1
[administrator] => 1
)

[filter] =>
)
[/php]

After writing this, I updated the WordPress Codex accordingly, which was missing all the 3.3.0 & 3.4.0 information.