Around a year ago I was playing with W3 Total Cache plugin on HHVM while I got an annoying warning
Warning: Parameter 1 to W3_Plugin_TotalCache::ob_callback() expected to be a reference, value given in /wp-includes/functions.php on line 3269
The funny part is that one of the functions was passing by reference which I still could not find the reason for it, maybe it was a limitation in PHP 4.3. Removing one character “&” fixed the issue so I submitted a pull request to the author although the repository currently does not accept pull request. Later on as PHP 7.0 was released, the issue started to show on PHP as well which brought more attention to this small fix. The users comments included anger such as user @kmob2 who said
this is becoming a running gag
Even more user @pratham2003 proposed a one line bash command to solve the problem
sed -i.bak 's/function ob_callback(&/function ob_callback(/g' /path/to/public_html/wp-content/plugins/w3-total-cache/lib/W3/Plugin/TotalCache.php
I hope that the developers will finally listen to the users and fix it soon!