Rule source on GitHub

DowngradePhp73JsonConstRector

Remove Json constant that available only in php 7.3

-$json = json_encode($content, JSON_THROW_ON_ERROR);
+$json = json_encode($content);
+if (json_last_error() !== JSON_ERROR_NONE) {
+    throw new \Exception(json_last_error_msg());
+}

-$content = json_decode($json, null, 512, JSON_THROW_ON_ERROR);
+$content = json_decode($json, null, 512);
+if (json_last_error() !== JSON_ERROR_NONE) {
+    throw new \Exception(json_last_error_msg());
+}

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp73\Rector\ConstFetch\DowngradePhp73JsonConstRector;

return RectorConfig::configure()
    ->withRules([
        DowngradePhp73JsonConstRector::class,
    ]);