Rule source on GitHub

DowngradeJsonDecodeNullAssociativeArgRector

Downgrade json_decode() with null associative argument function

 function exactlyNull(string $json)
 {
-    $value = json_decode($json, null);
+    $value = json_decode($json, true);
 }

 function possiblyNull(string $json, ?bool $associative)
 {
-    $value = json_decode($json, $associative);
+    $value = json_decode($json, $associative === null ?: $associative);
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp72\Rector\FuncCall\DowngradeJsonDecodeNullAssociativeArgRector;

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