Rule source on GitHub

DowngradeStrContainsRector

Replace str_contains() with strpos() !== false

 class SomeClass
 {
     public function run()
     {
-        return str_contains('abc', 'a');
+        return strpos('abc', 'a') !== false;
     }
 }

Configure your rector.php:

<?php

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

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