| auth_redirect() | Checks if a user is logged in, if not it redirects them to the login page. |
| count_many_users_posts() | Number of posts written by a list of users. |
| count_user_posts() | Number of posts user has written. |
| count_users() | Count number of users who have each of the user roles. |
| email_exists() | Checks whether the given email exists. |
| get_author_posts_url() | Retrieve the URL to the author page for the user with the ID provided. |
| get_current_user_id() | Get the current user's ID. |
| get_the_author() | Retrieve the author of the current post. |
| get_the_author_link() | Retrieve either author's link or author's name. |
| get_the_modified_author() | Retrieve the author who last edited the current post. |
| get_user_by() | Retrieve user info by a given field |
| get_userdata() | Retrieve user info by user ID. |
| get_users() | Retrieve list of users matching criteria. |
| is_admin_bar_showing() | Determine whether the admin bar should be showing. |
| sanitize_user() | Sanitizes a username, stripping out unsafe characters. |
| the_author_link() | Display either author's link or author's name. |
| the_author_posts() | Display the number of posts by the author of the current post. |
| the_author_posts_link() | Displays an HTML link to the author page of the current post's author. |
| the_modified_author() | Display the name of the author who last edited the current post, if the author's ID is available. |
| username_exists() | Checks whether the given username (login) exists. |
| validate_username() | Checks whether a username is valid. |
| wp_dropdown_roles() | Print out option html elements for role selectors. |
| wp_dropdown_users() | Create dropdown HTML content of users. |
| wp_get_current_user() | Retrieve the current user object. |
| wp_list_authors() | List all the authors of the site, with several options available. |
| wp_set_current_user() | Changes the current user by ID or name. The function does not authorize the user but changes the global variable |