$current_user or $user_ID global in main plugin file

I’m having some trouble pulling in the $current_user or $user_ID global in my main plug in file.

a var dump reveals that neither global is set at this point in the plugin. But it seems like the globals are available in the callback hooks. Help?

Code:

global $wpdb, $current_user, $user_ID;

if ( is_admin() )
{
	function myplugin_activation ()
	{
		global $wpdb;
		include ( WP_PLUGIN_DIR . '/soapblocks/php/activate.php' );
	}

	function myplugin_deactivate ()
	{
		global $wpdb;
		include ( WP_PLUGIN_DIR . '/mypluginmyplugin/php/deactivate.php' );
	}
	register_activation_hook( __FILE__ , 'myplugin_activation' );
	register_deactivation_hook ( __FILE__, 'myplugin_deactivate' );

	if ( isset ( $_GET['myplugin_post_id'] ) && is_numeric ( $_GET['myplugin_post_id'] ) )
	{
		$post_id = intval( $_GET['myplugin_post_id'] );

		$sql = 'SELECT * FROM ' . $wpdb->prefix . 'myplugin_my_table WHERE my_table_post_id = %d';

		$result = $wpdb->get_row( $wpdb->prepare( $sql, $post_id ) );

		if ( !$result )
		{
			$sql = 'INSERT INTO ' . $wpdb->prefix . 'myplugin_my_table ( my_table_post_id, my_table_user_id) VALUES ( %d, %d )';

			// here is where I'm trying to use the globals
			$wpdb->query ( $wpdb->prepare ( $sql, $post_id, $user_ID ) );
			header('Location: '.preg_replace("/\?myplugin_(?:action)=[0-9]+[&]?/", '', $_SERVER['REQUEST_URI']));
		}
	}
}