diff --git a/src/Cancellation.php b/src/Cancellation.php
index 6aece720..51f56449 100644
--- a/src/Cancellation.php
+++ b/src/Cancellation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/CancelledException.php b/src/CancelledException.php
index c5b5251d..8f21b8f9 100644
--- a/src/CancelledException.php
+++ b/src/CancelledException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/Closable.php b/src/Closable.php
index f8ebe67b..e8ffedce 100644
--- a/src/Closable.php
+++ b/src/Closable.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/CompositeCancellation.php b/src/CompositeCancellation.php
index b39e0fae..9e428873 100644
--- a/src/CompositeCancellation.php
+++ b/src/CompositeCancellation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/CompositeException.php b/src/CompositeException.php
index 522ff478..1093ba56 100644
--- a/src/CompositeException.php
+++ b/src/CompositeException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/CompositeLengthException.php b/src/CompositeLengthException.php
index e5e90a38..225936aa 100644
--- a/src/CompositeLengthException.php
+++ b/src/CompositeLengthException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/DeferredCancellation.php b/src/DeferredCancellation.php
index 4d9d56ea..c059ef4e 100644
--- a/src/DeferredCancellation.php
+++ b/src/DeferredCancellation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/DeferredFuture.php b/src/DeferredFuture.php
index 38f8ee6b..e45a582e 100644
--- a/src/DeferredFuture.php
+++ b/src/DeferredFuture.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/ForbidCloning.php b/src/ForbidCloning.php
index 92c588f4..3dd69a72 100644
--- a/src/ForbidCloning.php
+++ b/src/ForbidCloning.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/ForbidSerialization.php b/src/ForbidSerialization.php
index d8669a1d..fed8478e 100644
--- a/src/ForbidSerialization.php
+++ b/src/ForbidSerialization.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/Future.php b/src/Future.php
index 19a59abf..dccae481 100644
--- a/src/Future.php
+++ b/src/Future.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/Future/UnhandledFutureError.php b/src/Future/UnhandledFutureError.php
index b8b818ef..19e1d96e 100644
--- a/src/Future/UnhandledFutureError.php
+++ b/src/Future/UnhandledFutureError.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/src/Future/functions.php b/src/Future/functions.php
index c6f13f38..b3afa63e 100644
--- a/src/Future/functions.php
+++ b/src/Future/functions.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/src/Internal/Cancellable.php b/src/Internal/Cancellable.php
index 677bda17..2a4b0c78 100644
--- a/src/Internal/Cancellable.php
+++ b/src/Internal/Cancellable.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Internal;
 
diff --git a/src/Internal/FutureIterator.php b/src/Internal/FutureIterator.php
index db92099f..78d7ebe2 100644
--- a/src/Internal/FutureIterator.php
+++ b/src/Internal/FutureIterator.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Internal;
 
diff --git a/src/Internal/FutureIteratorQueue.php b/src/Internal/FutureIteratorQueue.php
index a018e010..c6d33c15 100644
--- a/src/Internal/FutureIteratorQueue.php
+++ b/src/Internal/FutureIteratorQueue.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Internal;
 
diff --git a/src/Internal/FutureState.php b/src/Internal/FutureState.php
index 42211eeb..1002634f 100644
--- a/src/Internal/FutureState.php
+++ b/src/Internal/FutureState.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Internal;
 
diff --git a/src/Internal/WrappedCancellation.php b/src/Internal/WrappedCancellation.php
index bdf1d375..84395194 100644
--- a/src/Internal/WrappedCancellation.php
+++ b/src/Internal/WrappedCancellation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Internal;
 
diff --git a/src/Internal/functions.php b/src/Internal/functions.php
index e0a9d6f8..3335a015 100644
--- a/src/Internal/functions.php
+++ b/src/Internal/functions.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Internal;
 
diff --git a/src/NullCancellation.php b/src/NullCancellation.php
index cd4e80a8..77e5c566 100644
--- a/src/NullCancellation.php
+++ b/src/NullCancellation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/SignalCancellation.php b/src/SignalCancellation.php
index 488ec6f3..133709ae 100644
--- a/src/SignalCancellation.php
+++ b/src/SignalCancellation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/SignalException.php b/src/SignalException.php
index 191c8a03..3ca48798 100644
--- a/src/SignalException.php
+++ b/src/SignalException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/TimeoutCancellation.php b/src/TimeoutCancellation.php
index 57e3f54e..a73f737c 100644
--- a/src/TimeoutCancellation.php
+++ b/src/TimeoutCancellation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/TimeoutException.php b/src/TimeoutException.php
index eece40f6..c995684b 100644
--- a/src/TimeoutException.php
+++ b/src/TimeoutException.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/src/functions.php b/src/functions.php
index e5f239d4..6962e911 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/test/CallbackStub.php b/test/CallbackStub.php
index 152e35be..0b75bffe 100644
--- a/test/CallbackStub.php
+++ b/test/CallbackStub.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/test/Cancellation/CancellationTest.php b/test/Cancellation/CancellationTest.php
index e9beb4b1..b2bf62e2 100644
--- a/test/Cancellation/CancellationTest.php
+++ b/test/Cancellation/CancellationTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Cancellation;
 
diff --git a/test/Cancellation/CompositeCancellationTest.php b/test/Cancellation/CompositeCancellationTest.php
index 3b317254..c005dc9e 100644
--- a/test/Cancellation/CompositeCancellationTest.php
+++ b/test/Cancellation/CompositeCancellationTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Cancellation;
 
diff --git a/test/Cancellation/SignalCancellationTest.php b/test/Cancellation/SignalCancellationTest.php
index 0dac0bfa..df057e4c 100644
--- a/test/Cancellation/SignalCancellationTest.php
+++ b/test/Cancellation/SignalCancellationTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Cancellation;
 
diff --git a/test/Cancellation/TimeoutCancellationTest.php b/test/Cancellation/TimeoutCancellationTest.php
index 28bd1f61..3b8e229f 100644
--- a/test/Cancellation/TimeoutCancellationTest.php
+++ b/test/Cancellation/TimeoutCancellationTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Cancellation;
 
diff --git a/test/Future/AwaitAllTest.php b/test/Future/AwaitAllTest.php
index 7e2e97f7..d65e653e 100644
--- a/test/Future/AwaitAllTest.php
+++ b/test/Future/AwaitAllTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/test/Future/AwaitAnyNTest.php b/test/Future/AwaitAnyNTest.php
index 81daea59..bff7f65f 100644
--- a/test/Future/AwaitAnyNTest.php
+++ b/test/Future/AwaitAnyNTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/test/Future/AwaitAnyTest.php b/test/Future/AwaitAnyTest.php
index bc87ce3f..35e45c55 100644
--- a/test/Future/AwaitAnyTest.php
+++ b/test/Future/AwaitAnyTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/test/Future/AwaitFirstTest.php b/test/Future/AwaitFirstTest.php
index f0160a03..686cc18d 100644
--- a/test/Future/AwaitFirstTest.php
+++ b/test/Future/AwaitFirstTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/test/Future/AwaitTest.php b/test/Future/AwaitTest.php
index ab4fd10f..d5fbfa02 100644
--- a/test/Future/AwaitTest.php
+++ b/test/Future/AwaitTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/test/Future/FutureTest.php b/test/Future/FutureTest.php
index 2f7078de..bef733d6 100644
--- a/test/Future/FutureTest.php
+++ b/test/Future/FutureTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp\Future;
 
diff --git a/test/TestCase.php b/test/TestCase.php
index 2d09db43..0f6e91b0 100644
--- a/test/TestCase.php
+++ b/test/TestCase.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;
 
diff --git a/test/WeakClosureTest.php b/test/WeakClosureTest.php
index 3d64a773..e7e4b616 100644
--- a/test/WeakClosureTest.php
+++ b/test/WeakClosureTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Amp;