добавил toString трансформер

src/Transformer/ToString.php

@@ -0,0 +1,14 @@
+<?php
+declare(strict_types=1);
+
+namespace Rinsvent\Transformer\Transformer;
+
+#[\Attribute(\Attribute::TARGET_ALL|\Attribute::IS_REPEATABLE)]
+class ToString extends Meta
+{
+    public function __construct(
+        public array $tags = ['default'],
+    ) {
+        parent::__construct(...func_get_args());
+    }
+}

src/Transformer/ToStringTransformer.php

@@ -0,0 +1,26 @@
+<?php
+declare(strict_types=1);
+
+namespace Rinsvent\Transformer\Transformer;
+
+use function PHPUnit\Framework\equalTo;
+
+class ToStringTransformer implements TransformerInterface
+{
+    /**
+     * @param string|null $data
+     * @param ToString $meta
+     * @return string|null
+     */
+    public function transform(mixed $data, Meta $meta): mixed
+    {
+        if ($data === null) {
+            return $data;
+        }
+        try {
+            return (string)$data;
+        } catch (\Throwable) {
+            return $data;
+        }
+    }
+}

tests/unit/ToStringTransformTest.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Rinsvent\DTO2Data\Tests\Converter;
+
+use tests\unit\fixtures\Foo;
+use Rinsvent\Transformer\Transformer;
+
+class ToStringTransformTest extends \Codeception\Test\Unit
+{
+    /**
+     * @var \UnitTester
+     */
+    protected $tester;
+
+    // tests
+    public function testSuccessFillRequestData()
+    {
+        $t = new Transformer();
+        $actual = $t->transform(new Foo(), new Transformer\ToString());
+        $this->assertEquals('to_string', $actual);
+    }
+}

tests/unit/fixtures/Foo.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace tests\unit\fixtures;
+
+class Foo
+{
+    public function __toString(): string
+    {
+        return 'to_string';
+    }
+}