diff --git a/src/Exception/CommonProblemDetailsExceptionTrait.php b/src/Exception/CommonProblemDetailsExceptionTrait.php
index 4d81fc2..2632729 100644
--- a/src/Exception/CommonProblemDetailsExceptionTrait.php
+++ b/src/Exception/CommonProblemDetailsExceptionTrait.php
@@ -9,6 +9,8 @@
 
 namespace Zend\ProblemDetails\Exception;
 
+use function array_merge;
+
 /**
  * Common functionality for ProblemDetailsExceptionInterface implementations.
  *
diff --git a/src/ProblemDetailsMiddleware.php b/src/ProblemDetailsMiddleware.php
index 1251c2c..a825b9b 100644
--- a/src/ProblemDetailsMiddleware.php
+++ b/src/ProblemDetailsMiddleware.php
@@ -17,6 +17,12 @@
 use Psr\Http\Server\RequestHandlerInterface;
 use Throwable;
 
+use function array_walk;
+use function error_reporting;
+use function in_array;
+use function restore_error_handler;
+use function set_error_handler;
+
 /**
  * Middleware that ensures a Problem Details response is returned
  * for all errors and Exceptions/Throwables.
@@ -76,7 +82,7 @@ public function process(ServerRequestInterface $request, RequestHandlerInterface
      */
     public function attachListener(callable $listener) : void
     {
-        if (\in_array($listener, $this->listeners, true)) {
+        if (in_array($listener, $this->listeners, true)) {
             return;
         }
 
diff --git a/src/ProblemDetailsNotFoundHandler.php b/src/ProblemDetailsNotFoundHandler.php
index 7d018e8..6462f25 100644
--- a/src/ProblemDetailsNotFoundHandler.php
+++ b/src/ProblemDetailsNotFoundHandler.php
@@ -15,6 +15,8 @@
 use Psr\Http\Server\MiddlewareInterface;
 use Psr\Http\Server\RequestHandlerInterface;
 
+use function sprintf;
+
 class ProblemDetailsNotFoundHandler implements MiddlewareInterface
 {
     /**
diff --git a/src/ProblemDetailsResponseFactory.php b/src/ProblemDetailsResponseFactory.php
index 674d2a4..d217e10 100644
--- a/src/ProblemDetailsResponseFactory.php
+++ b/src/ProblemDetailsResponseFactory.php
@@ -14,10 +14,28 @@
 use Negotiation\Negotiator;
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
-use Psr\Http\Message\StreamInterface;
 use Spatie\ArrayToXml\ArrayToXml;
 use Throwable;
 
+use function array_merge;
+use function array_walk_recursive;
+use function get_class;
+use function get_resource_type;
+use function is_array;
+use function is_int;
+use function is_resource;
+use function json_decode;
+use function json_encode;
+use function preg_replace;
+use function print_r;
+use function sprintf;
+use function strpos;
+
+use const JSON_PRESERVE_ZERO_FRACTION;
+use const JSON_PRETTY_PRINT;
+use const JSON_UNESCAPED_SLASHES;
+use const JSON_UNESCAPED_UNICODE;
+
 /**
  * Create a Problem Details response.
  *
diff --git a/test/Exception/ProblemDetailsExceptionInterfaceTest.php b/test/Exception/ProblemDetailsExceptionInterfaceTest.php
index 92162d6..ec9980f 100644
--- a/test/Exception/ProblemDetailsExceptionInterfaceTest.php
+++ b/test/Exception/ProblemDetailsExceptionInterfaceTest.php
@@ -13,6 +13,9 @@
 use Zend\ProblemDetails\Exception\CommonProblemDetailsExceptionTrait;
 use Zend\ProblemDetails\Exception\ProblemDetailsExceptionInterface;
 
+use function json_decode;
+use function json_encode;
+
 class ProblemDetailsExceptionTest extends TestCase
 {
     protected $status = 403;
diff --git a/test/ProblemDetailsAssertionsTrait.php b/test/ProblemDetailsAssertionsTrait.php
index 4c2a1c8..7ff43eb 100644
--- a/test/ProblemDetailsAssertionsTrait.php
+++ b/test/ProblemDetailsAssertionsTrait.php
@@ -12,10 +12,17 @@
 use PHPUnit\Framework\Assert;
 use Prophecy\Argument;
 use Prophecy\Prophecy\ObjectProphecy;
-use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\StreamInterface;
 use Throwable;
 
+use function array_walk_recursive;
+use function get_class;
+use function json_decode;
+use function json_encode;
+use function simplexml_load_string;
+use function sprintf;
+use function var_export;
+
 trait ProblemDetailsAssertionsTrait
 {
     public function assertProblemDetails(array $expected, array $details) : void
diff --git a/test/ProblemDetailsMiddlewareTest.php b/test/ProblemDetailsMiddlewareTest.php
index 9f48afe..58e46c7 100644
--- a/test/ProblemDetailsMiddlewareTest.php
+++ b/test/ProblemDetailsMiddlewareTest.php
@@ -17,7 +17,10 @@
 use Psr\Http\Server\RequestHandlerInterface;
 use Zend\ProblemDetails\ProblemDetailsMiddleware;
 use Zend\ProblemDetails\ProblemDetailsResponseFactory;
-use ZendTest\ProblemDetails\TestAsset;
+
+use function trigger_error;
+
+use const E_USER_ERROR;
 
 class ProblemDetailsMiddlewareTest extends TestCase
 {
@@ -90,14 +93,14 @@ public function testMiddlewareRegistersErrorHandlerToConvertErrorsToProblemDetai
         $handler
             ->handle(Argument::that([$this->request, 'reveal']))
             ->will(function () {
-                trigger_error('Triggered error!', \E_USER_ERROR);
+                trigger_error('Triggered error!', E_USER_ERROR);
             });
 
         $expected = $this->prophesize(ResponseInterface::class)->reveal();
         $this->responseFactory
             ->createResponseFromThrowable($this->request->reveal(), Argument::that(function ($e) {
                 $this->assertInstanceOf(ErrorException::class, $e);
-                $this->assertEquals(\E_USER_ERROR, $e->getSeverity());
+                $this->assertEquals(E_USER_ERROR, $e->getSeverity());
                 $this->assertEquals('Triggered error!', $e->getMessage());
                 return true;
             }))
diff --git a/test/ProblemDetailsResponseFactoryFactoryTest.php b/test/ProblemDetailsResponseFactoryFactoryTest.php
index 98652c1..76eea1a 100644
--- a/test/ProblemDetailsResponseFactoryFactoryTest.php
+++ b/test/ProblemDetailsResponseFactoryFactoryTest.php
@@ -21,6 +21,11 @@
 use Zend\ProblemDetails\ProblemDetailsResponseFactory;
 use Zend\ProblemDetails\ProblemDetailsResponseFactoryFactory;
 
+use const JSON_PRESERVE_ZERO_FRACTION;
+use const JSON_PRETTY_PRINT;
+use const JSON_UNESCAPED_SLASHES;
+use const JSON_UNESCAPED_UNICODE;
+
 class ProblemDetailsResponseFactoryFactoryTest extends TestCase
 {
     protected function setUp() : void
diff --git a/test/ProblemDetailsResponseFactoryTest.php b/test/ProblemDetailsResponseFactoryTest.php
index 4ef501a..358eb58 100644
--- a/test/ProblemDetailsResponseFactoryTest.php
+++ b/test/ProblemDetailsResponseFactoryTest.php
@@ -18,10 +18,14 @@
 use Psr\Http\Message\ServerRequestInterface;
 use Psr\Http\Message\StreamInterface;
 use RuntimeException;
-use Zend\ProblemDetails\Exception\InvalidResponseBodyException;
 use Zend\ProblemDetails\Exception\ProblemDetailsExceptionInterface;
 use Zend\ProblemDetails\ProblemDetailsResponseFactory;
 
+use function array_keys;
+use function fclose;
+use function fopen;
+use function stripos;
+
 class ProblemDetailsResponseFactoryTest extends TestCase
 {
     use ProblemDetailsAssertionsTrait;