After switching to PHP 7 you may find one or more warnings on your WordPress pages in this pattern:
Warning: Parameter 1 to [plugin name]::[function name]() expected to be a reference, value given in [absolute path to/wp-includes/functions.php on line [number]
Fixing is easy. Just go to your plugin and search/find the first [function name] – see above. This may lock like this:
function [function name](&$buffer) {…now delete the „&“
function [function name]($buffer) {Done :-)
PS: Of course this is only quick&dirty to make the website run. Don’t forget to inform the plugin author(s)!