Rule source on GitHub

DowngradeDereferenceableOperationRector

Add parentheses around non-dereferenceable expressions.

 function getFirstChar(string $str, string $suffix = '')
 {
-    return "$str$suffix"[0];
+    return ("$str$suffix")[0];
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\ArrayDimFetch\DowngradeDereferenceableOperationRector;

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