Wordpress - get_current_user_id() returns 0?

Going by wp_get_current_user() information in the Codex, the function utilizes the global $current_user object and if necessary initializes it before use. As others have stated, get_current_user_id() uses this function on the backend.

Consider /wp-includes/user.php, lines 323-327 (the function definition for this code). At the tail end, the return value is return ( isset( $user->ID ) ? (int) $user->ID : 0 ); — this code will return 0 if the logged-in user is somehow unavailable.

Use the init or any subsequent action to call this function. Calling it outside of an action can lead to troubles. See #14024 for details.

This comes from the documentation for wp_get_current_user. If you were using this code within a template, you could be assured that init had already been called. However, if you just randomly grab for that user information before init action is called, you will not get a current user. This explains why you got the User's ID when adding those actions into functions.php (as those actions take place after init), whereas with your original code it's not clear when you invoke that.

Please refer to the Plugin API page for a general idea of the order in which these various actions are invoked.

Always use get_current_user_id(); under init action.

Following is the example:

add_action('init', 'myFunction');

function myFunction(){

 $user_ID= get_current_user_id();   

   echo"User number $user_ID is loggedin";
}

As mentioned by the others: If you call the function too early it will return value 0

A good way to check if it's "too early" or not is this kind of checking:

// Do NOT check for action 'set_current_user', but for 'init'!!
if ( ! did_action( 'init' ) ) {
    _doing_it_wrong( __FUNCTION__, 'get_current_user_id() called before the init hook', null );
}
$user_id = get_current_user_id();

The reason why we do not use did_action('set_current_user') is:

If some other code/other plugin called get_current_user_id() too early, it will trigger the hook set_current_user to run. However, the current-user data is not correct at this point, so relying on that action hook is no good idea - only when init is executed we can be sure to have the correct user!