Also having this error filling the logs on HHVM.
Seems that it’s not that difficult to fix, according to this thread: https://ww.wp.xz.cn/support/topic/warning-with-hhvm?replies=19
But it is not fixed in the current version, is it?
Same with PHP 7.0 (Ubuntu 16.04 / Apache2.4 behind NGINX)
I had the same error. The solution is very simple actually.. You just need to remove the & character –
http://webhostingground.com/t/how-to-fix-w3-total-cache-error-on-wordpress
Although that fix certainly made the warning message disappear, I wonder if W3TC is working at all under PHP 7. I’m just asking, because those ob_callbacks() are very likely the glue that is behind the whole page capture and saving to memory or disk; in my case, I have noticed that W3TC simply refuses to write anything to disk. Ever again. Silly me, I had stale files lying around in those cache directories, from before the upgrade to PHP 7, and now, as far as I can see, they are not deleted, nor overwritten. Nothing. I can even delete the whole directory structure and W3TC will not write it again, ever.
It might be just my configuration, though.
