diff --git a/tests/WebToPay/Config/EnvReaderTest.php b/tests/WebToPay/Config/EnvReaderTest.php
new file mode 100644
index 0000000..e4b198c
--- /dev/null
+++ b/tests/WebToPay/Config/EnvReaderTest.php
@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+use PHPUnit\Framework\TestCase;
+
+class WebToPay_EnvReaderTest extends TestCase
+{
+    /**
+     * @dataProvider getAsArrayData
+     */
+    public function testGetAsString(string $key, ?string $expected, ?string $actual): void
+    {
+        $this->assertEquals($expected, $actual);
+    }
+
+    public function getAsArrayData(): iterable
+    {
+        $envReader = new WebToPay_EnvReader();
+
+        $key = 'testVar';
+        unset($_ENV[$key]);
+        putenv($key . '=');
+
+        yield 'not exists' => [
+            $key,
+            null,
+            $envReader->getAsString($key),
+        ];
+
+        $expected = 'defaultValue';
+
+        yield 'with default' => [
+            $key,
+            $expected,
+            $envReader->getAsString($key, $expected),
+        ];
+
+        $expected = 'testValue1';
+
+        $_ENV[$key] = $expected;
+
+        yield 'read from $_ENV' => [
+            $key,
+            $expected,
+            $envReader->getAsString($key),
+        ];
+
+        unset($_ENV[$key]);
+
+        putenv(sprintf('%s=%s', $key, $expected));
+
+        yield 'read from getenv()' => [
+            $key,
+            $expected,
+            $envReader->getAsString($key),
+        ];
+    }
+}