diff --git a/README.md b/README.md
index 905b396..05c7347 100644
--- a/README.md
+++ b/README.md
@@ -26,14 +26,95 @@ It also promises to be immutable after first read.
 
 ## Usage
 
+### Load config from file
+
+We support writing your config in php files
+
+Example: 
 ```php
+<?php
+
+// config.php
+
+return [
+	'config-key' => 'value',
+	'nested' => [
+		'key' => 'nested-value'
+		'bool-key' => false,
+	],
+];
+```
 
-$config = new \Stefna\Config\FileConfig('path-to-php-config.php');
+```php
+$config = new \Stefna\Config\FileConfig('path-to-php/config.php');
 // config file is not read until it's needed
 
-$config->getBool('boolKey');
+$config->getBool('nested.bool-key') === false;
+$config->getString('config-key') === 'value';
 ```
 
+### Load multiple files into config
+
+```php
+<?php
+
+// common.php
+
+return [
+	'config-key' => 'value',
+	'nested' => [
+		'key' => 'nested-value'
+		'bool-key' => false,
+	],
+];
+```
+```php
+<?php
+
+// production.php
+
+return [
+	'config-key' => 'production-value',
+	'nested' => [
+		'extra-key' => 42,
+	],
+];
+```
+
+```php
+$config = new \Stefna\Config\FileCollectionConfig('path-to-php/');
+$config->addFile('common.php');
+$config->addFile('production.php');
+
+// config files is not read until it's needed
+
+$config->getInt('nested.extra-key') === 42;
+$config->getString('config-key') === 'product-value';
+```
+
+### Mutable config
+
+We do provide a mutable config that allows you to override values in the "root" config
+this is meant to be used when testing applications but still allow the "root" configuration to stay immutable
+
+
+```php
+$rootConfig = new \Stefna\Config\FileCollectionConfig('path-to-php/');
+$rootConfig->addFile('common.php');
+$rootConfig->addFile('production.php');
+
+$config = new \Stefna\Config\MutableConfig($rootConfig);
+
+$config->setConfigValue('config-key', 'overridden-value');
+
+$config->getString('config-key') === 'overridden-value';
+
+$config->resetConfigValue('config-key');
+
+$config->getString('config-key') === 'production-value';
+```
+
+
 ## Contribute
 
 We are always happy to receive bug/security reports and bug/security fixes
diff --git a/src/MutableConfig.php b/src/MutableConfig.php
new file mode 100644
index 0000000..efffb31
--- /dev/null
+++ b/src/MutableConfig.php
@@ -0,0 +1,33 @@
+<?php declare(strict_types=1);
+
+namespace Stefna\Config;
+
+final class MutableConfig implements Config
+{
+	use GetConfigTrait;
+
+	/** @var array<string, mixed> */
+	private array $config = [];
+
+	public function __construct(
+		private readonly Config $rootConfig,
+	) {}
+
+	public function getRawValue(string $key): mixed
+	{
+		if (isset($this->config[$key])) {
+			return $this->config[$key];
+		}
+		return $this->rootConfig->get($key);
+	}
+
+	public function setConfigValue(string $key, mixed $value): void
+	{
+		$this->config[$key] = $value;
+	}
+
+	public function resetConfigValue(string $key): void
+	{
+		unset($this->config[$key]);
+	}
+}
diff --git a/tests/AbstractConfigTestCase.php b/tests/AbstractConfigTestCase.php
new file mode 100644
index 0000000..3275aaa
--- /dev/null
+++ b/tests/AbstractConfigTestCase.php
@@ -0,0 +1,147 @@
+<?php declare(strict_types=1);
+
+namespace Stefna\Config\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Stefna\Config\ArrayConfig;
+use Stefna\Config\Config;
+use Stefna\Config\Tests\Stub\TestStub;
+
+abstract class AbstractConfigTestCase extends TestCase
+{
+	public function testGetArrayConfigAsConfigObject(): void
+	{
+		$config = $this->getConfig();
+		$obj = $config->getArrayAsObject('testArray', ArrayConfig::class);
+
+		$this->assertInstanceOf(ArrayConfig::class, $obj);
+		$this->assertSame(1, $obj->getInt('random'));
+	}
+
+	public function testGetArrayObject(): void
+	{
+		$config = $this->getConfig();
+
+		$obj = $config->getArrayAsObject('testArray', TestStub::class);
+
+		$this->assertInstanceOf(TestStub::class, $obj);
+		$this->assertSame(1, $obj->random);
+	}
+
+	public function testGetArrayObjectWithInvalidClass(): void
+	{
+		$config = $this->getConfig();
+
+		$this->expectException(\BadMethodCallException::class);
+
+		$config->getArrayAsObject('testArray', \ArrayObject::class);
+	}
+
+	public function testGetArray(): void
+	{
+		$config = $this->getConfig();
+
+		$arr = $config->getArray('testArray');
+
+		$this->assertIsArray($arr);
+		$this->assertSame('1', $arr['random']);
+	}
+
+	public function testGetArrayWithDefaultValue(): void
+	{
+		$config = $this->getConfig();
+
+		$arr = $config->getArray('not-found', ['number' => 1]);
+
+		$this->assertIsArray($arr);
+		$this->assertSame(1, $arr['number']);
+	}
+
+	public function testGetArrayWithInvalidValueReturnsEmptyArray(): void
+	{
+		$config = $this->getConfig();
+
+		$arr = $config->getArray('testString');
+		$this->assertEmpty($arr);
+	}
+
+	public function testGetArrayNotFoundEmptyArray(): void
+	{
+		$config = $this->getConfig();
+
+		$arr = $config->getArray('not-found');
+		$this->assertEmpty($arr);
+	}
+
+	public function testGet(): void
+	{
+		$config = $this->getConfig();
+
+		$this->assertSame('string', $config->get('testString'));
+	}
+
+	public function testGetWithDefault(): void
+	{
+		$config = $this->getConfig();
+
+		$this->assertSame(1, $config->get('not-found', 1));
+	}
+
+	public function testGetNestedValueByDotKey(): void
+	{
+		$config = $this->getConfig();
+
+		$this->assertSame(1, $config->getInt('deep.nesting.int'));
+		$this->assertSame('test', $config->getString('deep.nesting.string'));
+	}
+
+	public function testGetNestedValueByDotKeyNotFound(): void
+	{
+		$config = $this->getConfig();
+
+		$this->assertNull($config->getInt('deep.nesting.not-found.int'));
+	}
+
+	public function testGetNestedValueHasLowerPriorityThanExactMatch(): void
+	{
+		$config = $this->getConfig();
+
+		$this->assertSame(1, $config->get('nesting.test.value'));
+	}
+
+	public function testDotKeyReturnEmptyOnExtraDots(): void
+	{
+		$config = $this->getConfig();
+
+		$this->assertNull($config->get('nesting.'));
+		$this->assertNull($config->get('.nesting'));
+	}
+
+	protected function getDefaultArrayConfig(): ArrayConfig
+	{
+		return new ArrayConfig([
+			'testString' => 'string',
+			'testNumberAsString' => '43',
+			'testNumber' => 42,
+			'testBool' => true,
+			'testBoolAsString' => 'on',
+			'testArray' => [
+				'random' => '1',
+			],
+			'deep' => [
+				'nesting' => [
+					'int' => 1,
+					'string' => 'test',
+				],
+			],
+			'nesting.test.value' => 1,
+			'nesting' => [
+				'test' => [
+					'value' => 2,
+				],
+			],
+		]);
+	}
+
+	abstract protected function getConfig(): Config;
+}
diff --git a/tests/ArrayConfigTest.php b/tests/ArrayConfigTest.php
index 8c2409c..0a049c7 100644
--- a/tests/ArrayConfigTest.php
+++ b/tests/ArrayConfigTest.php
@@ -6,139 +6,10 @@
 use Stefna\Config\ArrayConfig;
 use Stefna\Config\Tests\Stub\TestStub;
 
-final class ArrayConfigTest extends TestCase
+final class ArrayConfigTest extends AbstractConfigTestCase
 {
-	public function testGetArrayConfigAsConfigObject(): void
-	{
-		$config = $this->getConfig();
-		$obj = $config->getArrayAsObject('testArray', ArrayConfig::class);
-
-		$this->assertInstanceOf(ArrayConfig::class, $obj);
-		$this->assertSame(1, $obj->getInt('random'));
-	}
-
-	public function testGetArrayObject(): void
-	{
-		$config = $this->getConfig();
-
-		$obj = $config->getArrayAsObject('testArray', TestStub::class);
-
-		$this->assertInstanceOf(TestStub::class, $obj);
-		$this->assertSame(1, $obj->random);
-	}
-
-	public function testGetArrayObjectWithInvalidClass(): void
-	{
-		$config = $this->getConfig();
-
-		$this->expectException(\BadMethodCallException::class);
-
-		$config->getArrayAsObject('testArray', \ArrayObject::class);
-	}
-
-	public function testGetArray(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('testArray');
-
-		$this->assertIsArray($arr);
-		$this->assertSame('1', $arr['random']);
-	}
-
-	public function testGetArrayWithDefaultValue(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('not-found', ['number' => 1]);
-
-		$this->assertIsArray($arr);
-		$this->assertSame(1, $arr['number']);
-	}
-
-	public function testGetArrayWithInvalidValueReturnsEmptyArray(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('testString');
-		$this->assertEmpty($arr);
-	}
-
-	public function testGetArrayNotFoundEmptyArray(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('not-found');
-		$this->assertEmpty($arr);
-	}
-
-	public function testGet(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame('string', $config->get('testString'));
-	}
-
-	public function testGetWithDefault(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame(1, $config->get('not-found', 1));
-	}
-
-	public function testGetNestedValueByDotKey(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame(1, $config->getInt('deep.nesting.int'));
-		$this->assertSame('test', $config->getString('deep.nesting.string'));
-	}
-
-	public function testGetNestedValueByDotKeyNotFound(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertNull($config->getInt('deep.nesting.not-found.int'));
-	}
-
-	public function testGetNestedValueHasLowerPriorityThanExactMatch(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame(1, $config->get('nesting.test.value'));
-	}
-
-	public function testDotKeyReturnEmptyOnExtraDots(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertNull($config->get('nesting.'));
-		$this->assertNull($config->get('.nesting'));
-	}
-
 	public function getConfig(): ArrayConfig
 	{
-		return new ArrayConfig([
-			'testString' => 'string',
-			'testNumberAsString' => '43',
-			'testNumber' => 42,
-			'testBool' => true,
-			'testBoolAsString' => 'on',
-			'testArray' => [
-				'random' => '1',
-			],
-			'deep' => [
-				'nesting' => [
-					'int' => 1,
-					'string' => 'test',
-				],
-			],
-			'nesting.test.value' => 1,
-			'nesting' => [
-				'test' => [
-					'value' => 2,
-				],
-			],
-		]);
+		return $this->getDefaultArrayConfig();
 	}
 }
diff --git a/tests/ChainConfigTest.php b/tests/ChainConfigTest.php
index bbc13c6..6b95f9f 100644
--- a/tests/ChainConfigTest.php
+++ b/tests/ChainConfigTest.php
@@ -7,107 +7,8 @@
 use Stefna\Config\ChainConfig;
 use Stefna\Config\Tests\Stub\TestStub;
 
-final class ChainConfigTest extends TestCase
+final class ChainConfigTest extends AbstractConfigTestCase
 {
-	public function testGetArrayObject(): void
-	{
-		$config = $this->getConfig();
-
-		$obj = $config->getArrayAsObject('testArray', TestStub::class);
-
-		$this->assertInstanceOf(TestStub::class, $obj);
-		$this->assertSame(1, $obj->random);
-	}
-
-	public function testGetArrayObjectWithInvalidClass(): void
-	{
-		$config = $this->getConfig();
-
-		$this->expectException(\BadMethodCallException::class);
-
-		$config->getArrayAsObject('testArray', \ArrayObject::class);
-	}
-
-	public function testGetArray(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('testArray');
-
-		$this->assertIsArray($arr);
-		$this->assertSame('1', $arr['random']);
-	}
-
-	public function testGetArrayWithDefaultValue(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('not-found', ['number' => 1]);
-
-		$this->assertIsArray($arr);
-		$this->assertSame(1, $arr['number']);
-	}
-
-	public function testGetArrayWithInvalidValueReturnsEmptyArray(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('testString');
-		$this->assertEmpty($arr);
-	}
-
-	public function testGetArrayNotFoundEmptyArray(): void
-	{
-		$config = $this->getConfig();
-
-		$arr = $config->getArray('not-found');
-		$this->assertEmpty($arr);
-	}
-
-	public function testGet(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame('string', $config->get('testString'));
-	}
-
-	public function testGetWithDefault(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame(1, $config->get('not-found', 1));
-	}
-
-	public function testGetNestedValueByDotKey(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame(1, $config->getInt('deep.nesting.int'));
-		$this->assertSame('test', $config->getString('deep.nesting.string'));
-	}
-
-	public function testGetNestedValueByDotKeyNotFound(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertNull($config->getInt('deep.nesting.not-found.int'));
-	}
-
-	public function testGetNestedValueHasLowerPriorityThanExactMatch(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertSame(1, $config->get('nesting.test.value'));
-	}
-
-	public function testDotKeyReturnEmptyOnExtraDots(): void
-	{
-		$config = $this->getConfig();
-
-		$this->assertNull($config->get('nesting.'));
-		$this->assertNull($config->get('.nesting'));
-	}
-
 	public function testGetValueFromSecondConfigArray(): void
 	{
 		$config = $this->getConfig();
@@ -119,28 +20,7 @@ public function testGetValueFromSecondConfigArray(): void
 	public function getConfig(): ChainConfig
 	{
 		return new ChainConfig(
-			new ArrayConfig([
-				'testString' => 'string',
-				'testNumberAsString' => '43',
-				'testNumber' => 42,
-				'testBool' => true,
-				'testBoolAsString' => 'on',
-				'testArray' => [
-					'random' => '1',
-				],
-				'deep' => [
-					'nesting' => [
-						'int' => 1,
-						'string' => 'test',
-					],
-				],
-				'nesting.test.value' => 1,
-				'nesting' => [
-					'test' => [
-						'value' => 2,
-					],
-				],
-			]),
+			$this->getDefaultArrayConfig(),
 			new ArrayConfig([
 				'secondConfig' => [
 					'nest' => true,
diff --git a/tests/MutableConfigTest.php b/tests/MutableConfigTest.php
new file mode 100644
index 0000000..9c9f534
--- /dev/null
+++ b/tests/MutableConfigTest.php
@@ -0,0 +1,59 @@
+<?php declare(strict_types=1);
+
+namespace Stefna\Config\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Stefna\Config\ArrayConfig;
+use Stefna\Config\MutableConfig;
+use Stefna\Config\Tests\Stub\TestStub;
+
+final class MutableConfigTest extends AbstractConfigTestCase
+{
+	public function testOverrideValue(): void
+	{
+		$config = $this->getConfig();
+		$key = 'testString';
+		$originalValue = $config->get($key);
+		$newValue = 'overrideValue';
+
+		$config->setConfigValue($key, $newValue);
+
+		$this->assertNotSame($originalValue, $newValue);
+		$this->assertSame($newValue, $config->getString($key));
+	}
+
+	public function testOverrideDotKeyValues(): void
+	{
+		$config = $this->getConfig();
+		$key = 'deep.nesting.int';
+		$originalValue = $config->get($key);
+		$newValue = 42;
+
+		$config->setConfigValue($key, $newValue);
+
+		$this->assertNotSame($originalValue, $newValue);
+		$this->assertSame($newValue, $config->getInt($key));
+	}
+
+	public function testOverrideTestValue(): void
+	{
+		$config = $this->getConfig();
+		$key = 'nesting.test';
+		$originalValue = $config->get($key);
+		$newValue = 'overrideValue';
+
+		$config->setConfigValue($key, $newValue);
+
+		$this->assertNotSame($originalValue, $newValue);
+		$this->assertSame($newValue, $config->get($key));
+
+		$config->resetConfigValue($key);
+
+		$this->assertSame($originalValue, $config->get($key));
+	}
+
+	public function getConfig(): MutableConfig
+	{
+		return new MutableConfig($this->getDefaultArrayConfig());
+	}
+}