PHP deprecated mcrypt in 7.1
Funny situation, more and more scripts needs to downgrade PHP version.
mysql was dropped earlier, now the same became with
mcrypt. So, "OC Store" has no adopted to PHP7, as "Open Cart" too.
How about PHP-FPM ver. 5.6.31? This version can exists with PHP7 together on one server and make migration between PHP versions more painless.