Rule source on GitHub

DowngradePhp72JsonConstRector

Remove Json constant that available only in php 7.2

-$inDecoder = new Decoder($connection, true, 512, \JSON_INVALID_UTF8_IGNORE);
+$inDecoder = new Decoder($connection, true, 512, 0);

-$inDecoder = new Decoder($connection, true, 512, \JSON_INVALID_UTF8_SUBSTITUTE);
+$inDecoder = new Decoder($connection, true, 512, 0);

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp72\Rector\ConstFetch\DowngradePhp72JsonConstRector;

return RectorConfig::configure()
    ->withRules([
        DowngradePhp72JsonConstRector::class,
    ]);