Weird error in log

Hello everybody, this is my first post in this forum.

My name is David and I've a company in Madrid related to ecommerce and web development.

I've being working with Laravel since a year, after being into PHP since 1998. I've already done a few sites with it (10+), and it's part of our main stack for webdev.

Today I've received an email from one app of mine app with a very strange message:

[2015-03-06 13:21:44] production.ERROR: exception 'Symfony\Component\Debug\Exception\FatalErrorException' with message 'Call to undefined function send_origin_headers()' in /var/www/blog/wp-admin/admin-ajax.php:25

This shouldn't ever happen, /var/www/blog is another site on the same machine, running Wordpress.

How did this file got "included" and "executed" on my project?

It seems I've being somehow hacked (!!!) but after revisiting the code and digging in Apache logs I cannot see anything strange that could to my code to include a file from another directory. Just a few GETs and POSTs on the log with common and normal operations. Nothing specially "crafted".

I could almost reproduce the error by hand creating a route with

include('/var/www/blog/wp-admin/admin-ajax.php');

but had to modify too "admin-ajax.php" by hand to get the very same error on the log.

Have you ever run into a problem like this? Any clue? Software (4.2.17 and packages) is up to date.

Thanks in advance. This is running me mad.