index_old_.php: Is my WP site hacked?

Hi folks,

I hosted my a few WP sites in a shared hosting. All of them are now displaying similar errors such as:

Warning: Unexpected character in input: '\' (ASCII=92) state=1 in /home/mitrakre/public_html/greenroompro.net/index.php on line 954

Warning: Unexpected character in input: '\' (ASCII=92) state=1 in /home/mitrakre/public_html/greenroompro.net/index.php on line 960

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /home/mitrakre/public_html/greenroompro.net/index.php:954) in /home/mitrakre/public_html/greenroompro.net/wp-content/plugins/si-contact-form/si-contact-form.php on line 940

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/mitrakre/public_html/greenroompro.net/index.php:954) in /home/mitrakre/public_html/greenroompro.net/wp-content/plugins/si-contact-form/si-contact-form.php on line 940

I am also unable to log into any of my sites.

When I checked the home folders, one by one, all of them contain index_old_.php in which are containing the default index.php scripts:

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require('./wp-blog-header.php');

. But the index.php file itself is now containing a very long scripts, 993 lines! If you want to see them, the file can be downloaded from https://www.4shared.com/file/x91yWRJUce/index_bak.html (I renamed it to index_bak.php).

I’ve tried to delete current index.php and rename the index_old_.php back to index.php. But it makes it worse instead.

Are my sites being hacked? I successfully restored on of my sites by deleting its db, folders and files, re-installing wp from beginning, and then importing sql from back up. Is there any way to restore them easier?

I really appreciate any suggestions. Thanks.