Rule source on GitHub

DowngradeSubstrFalsyRector

Downgrade substr() with cast string on possibly falsy result

-substr("a", 2);
+(string) substr("a", 2);

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\FuncCall\DowngradeSubstrFalsyRector;

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