所有PHP文件的第一行中的“<？php”每2-3周自动删除一次

We have two sites running on Wordpress and Symfony, hosted on Bluehost.com

Every 2-3 weeks, we see this following error on our site.

/** 
  * 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');

A freelance developer has told us that for some reason the first line of all php files <?php keeps getting deleted.

We then have to restore the sites from our backup.

Any thoughts/ideas/solutions would be much appreciated.