When developing custom themes and plugins for WordPress there are times you will need to get the logged in user’s ID. There are a few different ways to accomplish this, but I’m going to show you the easiest method. To get the user’s ID you’ll use the get_current_user_id() function like so:
$current_user_id = get_current_user_id(); echo 'Your User ID is: ' .$current_user_id;
The get_current_user_id() function will return the currently logged in user’s ID, or 0 if a user is not logged in. Another popular method, which requires a bit more code, is the get_currentuserinfo() function. The method I described above actually uses the wp_get_current_user() function, which is a wrapper for get_currentuserinfo(). So using get_current_user_id() is just a faster method for retrieving the same data. This function was added in WordPress 3.0 as part of the Multisite code merge into WordPress.
To learn more about the get_current_user_id() function check out the Codex article or consult the WordPress core.