Rule source on GitHub

DowngradeFirstClassCallableSyntaxRector

Replace variadic placeholders usage by Closure::fromCallable()

-$cb = strlen(...);
+$cb = \Closure::fromCallable('strlen');

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\FuncCall\DowngradeFirstClassCallableSyntaxRector;

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