Rule source on GitHub

DowngradeIsCountableRector

Downgrade is_countable() to former version

 $items = [];
-return is_countable($items);
+return is_array($items) || $items instanceof Countable;

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp73\Rector\FuncCall\DowngradeIsCountableRector;

return RectorConfig::configure()
    ->withRules([
        DowngradeIsCountableRector::class,
    ]);