Rule source on GitHub

DowngradeStringReturnTypeOnToStringRector

Add "string" return on current __toString() method when parent method has string return on __toString() method

 abstract class ParentClass
 {
     public function __toString(): string
     {
         return 'value';
     }
 }

 class ChildClass extends ParentClass
 {
-    public function __toString()
+    public function __toString(): string
     {
         return 'value';
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp80\Rector\ClassMethod\DowngradeStringReturnTypeOnToStringRector;

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