I am not sure what exactly is causing the problem here as this has never been reported. I will suggest 2 approaches. If the 1st approach does not solve the issue, you can follow the 2nd one.
1. Can you please take out the complete function definition for PutWtiLikePost from wti_like_post_site.php and put it inside wti_like_post.php file?
2. Please disable warning message on your server. You may try putting the following code in wp-config.php file
error_reporting(E_ALL ^ E_WARNING);