DeclareStrictTypesRector

Add declare(strict_types=1) if missing

+declare(strict_types=1);
+
 function someFunction()
 {
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\TypeDeclaration\Rector\StmtsAwareInterface\DeclareStrictTypesRector;

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