Retrieves the information pertaining to the currently logged in user, and places it in the global variable $current_user

This function does not accept any parameters.

Usage:

<?php wp_get_current_user(); ?> 

Example:

  <?php
  $current_user = wp_get_current_user();

  echo 'Username: ' . $current_user->user_login . "\n";
  echo 'User email: ' . $current_user->user_email . "\n";
  echo 'User level: ' . $current_user->user_level . "\n";
  echo 'User first name: ' . $current_user->user_firstname . "\n";
  echo 'User last name: ' . $current_user->user_lastname . "\n";
  echo 'User display name: ' . $current_user->display_name . "\n";
  echo 'User ID: ' . $current_user->ID . "\n";
  ?>

To determine if a visitor is logged in or not, you can use is_user_logged_in() before, then get the current user info if the visitor it's logged in:

<?php
if ( is_user_logged_in() ) {
    $current_user = wp_get_current_user();

    echo 'Welcome, ' . $current_user->user_login . '!';
} else {
    echo 'Welcome, visitor!';
}
?>