diff --git a/.travis.yml b/.travis.yml
index 8e60692..edb8a09 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,11 +1,13 @@
 language: php
-
+dist: trusty
 sudo: required
 
 php:
-  - 5.4
-  - 5.5
   - 5.6
+  - 7.1
+  - 7.2
+  - 7.3
+  - 7.4
 
 env: TEST_PARAMS='-c tests/php.ini'
 
diff --git a/composer.json b/composer.json
index c005229..6645a5c 100644
--- a/composer.json
+++ b/composer.json
@@ -10,13 +10,13 @@
     }
   ],
   "require": {
-    "php": ">=5.4",
-    "guzzlehttp/guzzle": "^5.3"
+    "php": ">=5.6",
+    "guzzlehttp/guzzle": "^6.2"
   },
   "require-dev": {
     "tracy/tracy": "^2.4",
     "nette/tester": "^1.7",
-    "mockery/mockery": "^0.9.9",
+    "mockery/mockery": "^1.3",
     "latte/latte": "^2.4"
   },
   "autoload": {
diff --git a/examples/clientApi.php b/examples/clientApi.php
index c0da779..44975ec 100644
--- a/examples/clientApi.php
+++ b/examples/clientApi.php
@@ -3,7 +3,7 @@
 require __DIR__ . '/bootstrap.php';
 
 use GuzzleHttp\Exception\BadResponseException;
-use GuzzleHttp\Message\Request;
+use GuzzleHttp\Psr7\Request;
 use SizeID\OAuth2\ClientApi;
 
 
diff --git a/examples/popupLogin.php b/examples/popupLogin.php
index cd77550..4b76b79 100644
--- a/examples/popupLogin.php
+++ b/examples/popupLogin.php
@@ -1,6 +1,6 @@
 <?php
 require __DIR__ . '/bootstrap.php';
-use GuzzleHttp\Message\Request;
+use GuzzleHttp\Psr7\Request;
 use Latte\Engine;
 use SizeID\OAuth2\Exceptions\RedirectException;
 use SizeID\OAuth2\Repositories\SessionAccessTokenRepository;
diff --git a/examples/userApi.php b/examples/userApi.php
index a90f30d..ca72e4a 100644
--- a/examples/userApi.php
+++ b/examples/userApi.php
@@ -1,7 +1,7 @@
 <?php
 require __DIR__ . '/bootstrap.php';
 use GuzzleHttp\Exception\BadResponseException;
-use GuzzleHttp\Message\Request;
+use GuzzleHttp\Psr7\Request;
 use SizeID\OAuth2\Exceptions\RedirectException;
 use SizeID\OAuth2\UserApi;
 
diff --git a/src/Api.php b/src/Api.php
index a4cd479..0ef86dc 100644
--- a/src/Api.php
+++ b/src/Api.php
@@ -5,11 +5,13 @@
 use GuzzleHttp\Client;
 use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Exception\ClientException;
-use GuzzleHttp\Message\RequestInterface;
-use GuzzleHttp\Message\ResponseInterface;
+use GuzzleHttp\Psr7\Uri;
+use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
 use SizeID\OAuth2\Entities\AccessToken;
 use SizeID\OAuth2\Exceptions\InvalidStateException;
 use SizeID\OAuth2\Repositories\AccessTokenRepositoryInterface;
+use GuzzleHttp\RequestOptions;
 
 /**
  * Shared functionality of API calls
@@ -59,19 +61,22 @@ public function __construct(
 		$authorizationServerUrl,
 		$apiBaseUrl,
 		$httpClient
-	)
-	{
+	) {
 		$this->clientId = $clientId;
 		$this->clientSecret = $clientSecret;
 		$this->accessTokenRepository = $accessTokenRepository;
-		if ($authorizationServerUrl === NULL) {
+		if ($authorizationServerUrl === null) {
 			$authorizationServerUrl = Config::AUTHORIZATION_SERVER_URL;
 		}
-		if ($apiBaseUrl === NULL) {
+		if ($apiBaseUrl === null) {
 			$apiBaseUrl = Config::API_URL;
 		}
-		if ($httpClient === NULL) {
-			$httpClient = new Client();
+		if ($httpClient === null) {
+			$httpClient = new Client(
+				[
+					RequestOptions::HTTP_ERRORS => false,
+				]
+			);
 		}
 		$this->authorizationServerUrl = $authorizationServerUrl;
 		$this->apiBaseUrl = $apiBaseUrl;
@@ -154,7 +159,7 @@ protected function parseToken(ResponseInterface $response)
 	private function createResponse(RequestInterface $request)
 	{
 		$response = $this->callApi($this->buildRequest($request));
-		if ($response->getStatusCode() === 401 && $response->getHeader(self::SIZEID_ERROR_CODE_HEADER) == 109) {
+		if ($response->getStatusCode() === 401 && $response->getHeaderLine(self::SIZEID_ERROR_CODE_HEADER) == 109) {
 			$this->refreshAccessToken();
 			return $this->callApi($this->buildRequest($request));
 		}
@@ -176,9 +181,7 @@ private function callApi(RequestInterface $request)
 	 */
 	private function buildRequest(RequestInterface $request)
 	{
-		$request = clone $request;
-		$request->addHeader('Authorization', 'Bearer ' . $this->getAccessToken()->getAccessToken());
-		$request->setUrl($this->apiBaseUrl . '/' . $request->getUrl());
-		return $request;
+		return $request->withAddedHeader('Authorization', 'Bearer ' . $this->getAccessToken()->getAccessToken())
+			->withUri(new Uri($this->apiBaseUrl . '/' . $request->getUri()));
 	}
 }
\ No newline at end of file
diff --git a/src/ClientApi.php b/src/ClientApi.php
index 7b8519e..9c6648d 100644
--- a/src/ClientApi.php
+++ b/src/ClientApi.php
@@ -46,7 +46,7 @@ public function acquireNewAccessToken()
 		$response = $this->httpClient->post(
 			$this->authorizationServerUrl . '/access-token',
 			[
-				'body' => [
+				'form_params' => [
 					'grant_type' => 'client_credentials',
 					'client_id' => $this->clientId,
 					'client_secret' => $this->clientSecret,
diff --git a/src/UserApi.php b/src/UserApi.php
index a740994..2a96194 100644
--- a/src/UserApi.php
+++ b/src/UserApi.php
@@ -4,8 +4,9 @@
 
 use GuzzleHttp\ClientInterface;
 use GuzzleHttp\Exception\ClientException;
-use GuzzleHttp\Message\ResponseInterface;
-use GuzzleHttp\Url;
+use Psr\Http\Message\ResponseInterface;
+use GuzzleHttp\Psr7\Uri;
+use Psr\Http\Message\UriInterface;
 use SizeID\OAuth2\Entities\AccessToken;
 use SizeID\OAuth2\Exceptions\InvalidCSRFTokenException;
 use SizeID\OAuth2\Exceptions\InvalidStateException;
@@ -69,16 +70,13 @@ public function __construct(
 	 */
 	public function getAuthorizationUrl()
 	{
-		$url = Url::fromString($this->authorizationServerUrl);
-		$url->setQuery(
-			[
-				'response_type' => 'code',
-				'client_id' => $this->clientId,
-				'redirect_uri' => $this->redirectUri,
-				'state' => $this->csrfTokenRepository->generateCSRFToken(),
-			]
-		);
-		return $url;
+		$url = new Uri($this->authorizationServerUrl);
+		return Uri::withQueryValues($url, [
+			'response_type' => 'code',
+			'client_id' => $this->clientId,
+			'redirect_uri' => $this->redirectUri,
+			'state' => $this->csrfTokenRepository->generateCSRFToken(),
+		]);
 	}
 
 	/**
@@ -149,7 +147,7 @@ public function refreshAccessToken()
 			$this->saveTokenFromResponse($response);
 		} catch (ClientException $ex) {
 			$response = $ex->getResponse();
-			$sizeIdErrorCode = (int)$response->getHeader(self::SIZEID_ERROR_CODE_HEADER);
+			$sizeIdErrorCode = (int)$response->getHeaderLine(self::SIZEID_ERROR_CODE_HEADER);
 			if ($response->getStatusCode() === 400 && $sizeIdErrorCode === 108) {
 				//refresh token expired
 				throw RedirectException::create(
diff --git a/tests/ApiTest.phpt b/tests/ApiTest.phpt
index 94f383a..240e9bb 100644
--- a/tests/ApiTest.phpt
+++ b/tests/ApiTest.phpt
@@ -2,8 +2,8 @@
 
 namespace SizeID\OAuth2\Tests;
 
-use GuzzleHttp\Message\Request;
-use GuzzleHttp\Message\Response;
+use GuzzleHttp\Psr7\Request;
+use GuzzleHttp\Psr7\Response;
 use Mockery as m;
 use SizeID\OAuth2\ClientApi;
 use SizeID\OAuth2\Entities\AccessToken;
@@ -20,7 +20,7 @@ class ApiTest extends TestCase
 		$tokenRepository = m::mock('SizeID\OAuth2\Repositories\SessionAccessTokenRepository');
 		$tokenRepository
 			->shouldReceive('hasAccessToken')
-			->andReturn(NULL);
+			->andReturn(null);
 		$clientApi = new ClientApi(
 			'clientId',
 			'clientSecret',
diff --git a/tests/ClientApiTest.phpt b/tests/ClientApiTest.phpt
index ff59a28..7b342ee 100644
--- a/tests/ClientApiTest.phpt
+++ b/tests/ClientApiTest.phpt
@@ -2,12 +2,14 @@
 
 namespace SizeID\OAuth2\Tests;
 
-use GuzzleHttp\Message\Request;
-use GuzzleHttp\Message\Response;
+use GuzzleHttp\Psr7\Request;
+use GuzzleHttp\Psr7\Response;
 use Mockery as m;
+use Psr\Http\Message\StreamInterface;
 use SizeID\OAuth2\Api;
 use SizeID\OAuth2\ClientApi;
 use SizeID\OAuth2\Entities\AccessToken;
+use SizeID\OAuth2\Repositories\SessionAccessTokenRepository;
 use Tester\Assert;
 use Tester\TestCase;
 
@@ -27,7 +29,7 @@ class ClientApiTest extends TestCase
 
 	public function testAcquireToken()
 	{
-		$tokenRepository = m::mock('SizeID\OAuth2\Repositories\SessionAccessTokenRepository');
+		$tokenRepository = m::mock(SessionAccessTokenRepository::class);
 		$tokenRepository
 			->shouldReceive('hasAccessToken')
 			->andReturn(FALSE);
@@ -38,7 +40,7 @@ class ClientApiTest extends TestCase
 			->shouldReceive('getAccessToken')
 			->andReturn($accessToken);
 		$httpClient = m::mock('GuzzleHttp\Client');
-		$stream = m::mock('GuzzleHttp\Stream\StreamInterface');
+		$stream = m::mock(StreamInterface::class);
 		$stream
 			->shouldReceive('getContents')
 			->andReturn('{"access_token":"token", "expires_in": 60}');
@@ -57,13 +59,13 @@ class ClientApiTest extends TestCase
 			NULL,
 			$httpClient
 		);
-		Assert::type('SizeID\OAuth2\ClientApi', $clientApi);
+		Assert::type(ClientApi::class, $clientApi);
 		$clientApi->send(new Request('get', 'client'));
 	}
 
 	public function testRefreshToken()
 	{
-		$tokenRepository = m::mock('SizeID\OAuth2\Repositories\SessionAccessTokenRepository');
+		$tokenRepository = m::mock(SessionAccessTokenRepository::class);
 		$tokenRepository
 			->shouldReceive('hasAccessToken')
 			->andReturn(TRUE);
@@ -78,7 +80,7 @@ class ClientApiTest extends TestCase
 		$httpClient
 			->shouldReceive('send')
 			->andReturn($response);
-		$stream = m::mock('GuzzleHttp\Stream\StreamInterface');
+		$stream = m::mock(StreamInterface::class);
 		$stream
 			->shouldReceive('getContents')
 			->andReturn('{"access_token":"token", "expires_in": 60}');
@@ -97,8 +99,8 @@ class ClientApiTest extends TestCase
 			NULL,
 			$httpClient
 		);
-		Assert::type('SizeID\OAuth2\ClientApi', $clientApi);
-		Assert::type('GuzzleHttp\Message\Response', $clientApi->send(new Request('POST', 'client')));
+		Assert::type(ClientApi::class, $clientApi);
+		Assert::type(Response::class, $clientApi->send(new Request('POST', 'client')));
 	}
 }
 
diff --git a/tests/UserApiTest.phpt b/tests/UserApiTest.phpt
index f6bc07d..5789437 100644
--- a/tests/UserApiTest.phpt
+++ b/tests/UserApiTest.phpt
@@ -2,12 +2,19 @@
 
 namespace SizeID\OAuth2\Tests;
 
-use GuzzleHttp\Message\Request;
-use GuzzleHttp\Message\Response;
+use GuzzleHttp\ClientInterface;
+use GuzzleHttp\Exception\ClientException;
+use GuzzleHttp\Psr7\Request;
+use GuzzleHttp\Psr7\Response;
 use Mockery as m;
+use Psr\Http\Message\StreamInterface;
 use SizeID\OAuth2\Api;
 use SizeID\OAuth2\Entities\AccessToken;
+use SizeID\OAuth2\Exceptions\InvalidCSRFTokenException;
+use SizeID\OAuth2\Exceptions\InvalidStateException;
 use SizeID\OAuth2\Exceptions\RedirectException;
+use SizeID\OAuth2\Repositories\AccessTokenRepositoryInterface;
+use SizeID\OAuth2\Repositories\CsrfTokenRepositoryInterface;
 use SizeID\OAuth2\UserApi;
 use Tester\Assert;
 use Tester\TestCase;
@@ -24,26 +31,26 @@ class UserApiTest extends TestCase
 			'clientSecret',
 			'http://9gag.com'
 		);
-		Assert::type('SizeID\OAuth2\UserApi', $clientApi);
+		Assert::type(UserApi::class, $clientApi);
 	}
 
 	public function testAuthorize()
 	{
-		$tokenRepository = m::mock('SizeID\OAuth2\Repositories\AccessTokenRepositoryInterface');
+		$tokenRepository = m::mock(AccessTokenRepositoryInterface::class);
 		$tokenRepository
 			->shouldReceive('hasAccessToken')
 			->once()
-			->andReturn(FALSE);
+			->andReturn(false);
 		$tokenRepository
 			->shouldReceive('saveAccessToken');
-		$stateRepository = m::mock('SizeID\OAuth2\Repositories\CsrfTokenRepositoryInterface');
+		$stateRepository = m::mock(CsrfTokenRepositoryInterface::class);
 		$stateRepository
 			->shouldReceive('generateCSRFToken')
 			->andReturn('csrfToken');
 		$stateRepository
 			->shouldReceive('loadTokenCSRFToken')
 			->andReturn('csrfToken');
-		$httpClient = m::mock('GuzzleHttp\ClientInterface');
+		$httpClient = m::mock(ClientInterface::class);
 		$userApi = new UserApi(
 			'clientId',
 			'clientSecret',
@@ -56,7 +63,7 @@ class UserApiTest extends TestCase
 		);
 		try {
 			$userApi->send(new Request('get', 'user'));
-			Assert::fail('SizeID\OAuth2\Exceptions\RedirectException' . ' should be thrown');
+			Assert::fail(RedirectException::class . ' should be thrown');
 		} catch (RedirectException $ex) {
 			Assert::equal(RedirectException::CODE_MISSING_TOKEN, $ex->getCode());
 			Assert::equal(
@@ -64,7 +71,7 @@ class UserApiTest extends TestCase
 				(string)$ex->getRedirectUrl()
 			);
 		}
-		$stream = m::mock('GuzzleHttp\Stream\StreamInterface');
+		$stream = m::mock(StreamInterface::class);
 		$stream
 			->shouldReceive('getContents')
 			->andReturn('{"access_token":"token", "expires_in": 60, "refresh_token": "refresh_token"}');
@@ -75,7 +82,7 @@ class UserApiTest extends TestCase
 		$userApi->completeAuthorization("authCode", "csrfToken");
 		$tokenRepository
 			->shouldReceive('hasAccessToken')
-			->andReturn(TRUE);
+			->andReturn(true);
 		$tokenRepository
 			->shouldReceive('deleteAccessToken');
 		$tokenRepository
@@ -86,7 +93,7 @@ class UserApiTest extends TestCase
 			function () use ($userApi) {
 				$userApi->send(new Request('get', 'user'));
 			},
-			'SizeID\OAuth2\Exceptions\InvalidStateException'
+			InvalidStateException::class
 		);
 		$tokenRepository
 			->shouldReceive('getAccessToken')
@@ -94,30 +101,30 @@ class UserApiTest extends TestCase
 		$httpClient
 			->shouldReceive('send')
 			->andReturn(new Response(200));
-		Assert::type('GuzzleHttp\Message\Response', $userApi->send(new Request('get', 'user')));
+		Assert::type(Response::class, $userApi->send(new Request('get', 'user')));
 	}
 
 	public function testRefreshToken()
 	{
-		$tokenRepository = m::mock('SizeID\OAuth2\Repositories\AccessTokenRepositoryInterface');
+		$tokenRepository = m::mock(AccessTokenRepositoryInterface::class);
 		$tokenRepository
 			->shouldReceive('getAccessToken')
-			->andReturn(new AccessToken('acessToken', 'refreshToken'));
+			->andReturn(new AccessToken('accessToken', 'refreshToken'));
 		$tokenRepository
 			->shouldReceive('saveAccessToken')
 			->with(
 				m::on(
 					function (AccessToken $accessToken) {
 						Assert::equal('newRefreshToken', $accessToken->getRefreshToken());
-						return TRUE;
+						return true;
 					}
 				)
 			);
-		$stream = m::mock('GuzzleHttp\Stream\StreamInterface');
+		$stream = m::mock(StreamInterface::class);
 		$stream
 			->shouldReceive('getContents')
 			->andReturn('{"access_token":"token", "expires_in": 60, "refresh_token": "newRefreshToken"}');
-		$httpClient = m::mock('GuzzleHttp\ClientInterface');
+		$httpClient = m::mock(ClientInterface::class);
 		$httpClient
 			->shouldReceive('post')
 			->andReturn(new Response(200, [], $stream));
@@ -129,18 +136,18 @@ class UserApiTest extends TestCase
 			'authServer',
 			'apiUrl',
 			$httpClient,
-			NULL
+			null
 		);
 		$userApi->refreshAccessToken();
 	}
 
 	public function testInvalidRefreshToken()
 	{
-		$tokenRepository = m::mock('SizeID\OAuth2\Repositories\AccessTokenRepositoryInterface');
+		$tokenRepository = m::mock(AccessTokenRepositoryInterface::class);
 		$tokenRepository
 			->shouldReceive('getAccessToken')
-			->andReturn(new AccessToken('acessToken', 'refreshToken'));
-		$responseException = m::mock('GuzzleHttp\Exception\ClientException');
+			->andReturn(new AccessToken('accessToken', 'refreshToken'));
+		$responseException = m::mock(ClientException::class);
 		$errorResponse = new Response(
 			400,
 			[Api::SIZEID_ERROR_CODE_HEADER => "108"]
@@ -148,7 +155,7 @@ class UserApiTest extends TestCase
 		$responseException
 			->shouldReceive('getResponse')
 			->andReturn($errorResponse);
-		$httpClient = m::mock('GuzzleHttp\ClientInterface');
+		$httpClient = m::mock(ClientInterface::class);
 		$httpClient
 			->shouldReceive('post')
 			->once()
@@ -161,18 +168,17 @@ class UserApiTest extends TestCase
 			'authServer',
 			'apiUrl',
 			$httpClient,
-			NULL
+			null
 		);
 		Assert::exception(
 			function () use ($userApi) {
 				$userApi->refreshAccessToken();
-			}
-			,
-			'SizeID\OAuth2\Exceptions\RedirectException',
-			NULL,
+			},
+			RedirectException::class,
+			null,
 			RedirectException::CODE_EXPIRED_REFRESH_TOKEN
 		);
-		$responseException = m::mock('GuzzleHttp\Exception\ClientException');
+		$responseException = m::mock(ClientException::class);
 		$errorResponse = new Response(
 			400
 		);
@@ -188,7 +194,7 @@ class UserApiTest extends TestCase
 				$userApi->refreshAccessToken();
 			}
 			,
-			'GuzzleHttp\Exception\ClientException'
+			ClientException::class
 		);
 	}
 
@@ -196,7 +202,7 @@ class UserApiTest extends TestCase
 	{
 		$_GET['code'] = 'code';
 		$_GET['state'] = 'state1';
-		$csrfTokenRepository = m::mock('SizeID\OAuth2\Repositories\CsrfTokenRepositoryInterface');
+		$csrfTokenRepository = m::mock(CsrfTokenRepositoryInterface::class);
 		$csrfTokenRepository
 			->shouldReceive('loadTokenCSRFToken')
 			->andReturn('state2');
@@ -204,17 +210,17 @@ class UserApiTest extends TestCase
 			'clientId',
 			'clientSecret',
 			'redirectUri',
-			NULL,
+			null,
 			'authServer',
 			'apiUrl',
-			NULL,
-			NULL
+			null,
+			null
 		);
 		Assert::exception(
 			function () use ($userApi) {
 				$userApi->completeAuthorization();
 			},
-			'SizeID\OAuth2\Exceptions\InvalidCSRFTokenException',
+			InvalidCSRFTokenException::class,
 			'Invalid CSRF token.'
 		);
 	}