The exact same thing happened to my site. The hosting provider is Clook, if that means anything. I wonder if they're somehow to blame for this?
Either way, I didn't update to the latest WP version for a long time because the web designer doesn't work on the site any more, and the last time it updated, the theme got messed up.
Right now I was able to restore a backup, but there's weird stuff happening to the site. I updated to latest WP now, but I think the hacked code in the header is still there.
"call_user_func_array() expects parameter 1 to be a valid callback, first array member is not a valid class name or object in...." is what's showing on the main page. and the stuff that's in the first line of every .php file is huge, I'm not sure what's real and what's not. removing it just breaks the site.