Rule source on GitHub

DowngradeStrStartsWithRector

Downgrade str_starts_with() to strncmp() version

-str_starts_with($haystack, $needle);
+strncmp($haystack, $needle, strlen($needle)) === 0;

Configure your rector.php:

<?php

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

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