This plugin can easy return the ID number of the current user.
You can use any page/post or any php file.
To use this plugin you just need to install the plugin then use the shortcut [show-me-id] or in the php file using the php function "php echo apply_shortcode("[show-me-id]");"
Also it comes with css class to customize, class="usr-id"
Another Feature is a verification if the user is logged, if doesn't the plugin return the message No logged user