This shows up regularly when testing on mwdebug, and seems to be specific to PHP 7.
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20151012/luasandbox.so' - /usr/lib/php/20151012/luasandbox.so: cannot open shared object file: No such file or directory in Unknown on line 0
type syslog host mwdebug1001 program php7.0
Searching through Phabricator, found one previous match: