Plugin uninstallation does not work!

Uninstalling a plugin after deleting it for some unknown reason does not work. This is my code:

function sb_uninstall_plugin(){
	global $wpdb, $sb_ip_log;

	delete_option("sb_detection_of_spammers_cookies");
	delete_option("sb_detection_of_spammers_time");
	delete_option("sb_banned_message");
	$wpdb->query("DROP TABLE $sb_ip_log");
}

register_uninstall_hook(__FILE__, 'sb_uninstall_plugin');

The value of the option uninstall_plugins in the table wp_options: a:2:{i:0;b:0;s:22:”sb/spammer-blocker.php”;s:19:”sb_uninstall_plugin”;}

I also tried using uninstall.php without any success.
Thank you for your help!