https://phabricator.wikimedia.org/T182177 describes several usages of "each" which is deprecated in PHP since 7.2. The Elastica dependencies on elasticsearch/elasticsearch and ruflin/elastica should be updated to 5.3.2 from 5.3.0 to pull in the fixes from upstream. Currently, the Elastica extension (REL1_31 in combination with Elasticsearch 5.6.x and PHP 7.2.10) emits this warning (when $wgDevelopmentWarnings = 1;):
Deprecated : The each() function is deprecated. This message will be suppressed on further calls in /srv/www/sites/mediawiki/extensions/Elastica/vendor/ruflin/elastica/lib/Elastica/Multi/MultiBuilder.php on line 50