Rule source on GitHub

ArrowFunctionToAnonymousFunctionRector

Replace arrow functions with anonymous functions

 class SomeClass
 {
     public function run()
     {
         $delimiter = ",";
-        $callable = fn($matches) => $delimiter . strtolower($matches[1]);
+        $callable = function ($matches) use ($delimiter) {
+            return $delimiter . strtolower($matches[1]);
+        };
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp74\Rector\ArrowFunction\ArrowFunctionToAnonymousFunctionRector;

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