diff --git a/src/Cron.php b/src/Cron.php
index 639957b..517a5ab 100644
--- a/src/Cron.php
+++ b/src/Cron.php
@@ -56,11 +56,17 @@ public function isDue(DateTimeInterface $dateTime): bool
     public function getNextDue(DateTimeInterface $dateTime): DateTimeInterface
     {
         if ($this->isExpired($dateTime)) {
-            return $this->end;
+            /** @var DateTimeInterface $end */
+            $end = $this->end;
+
+            return $end;
         }
 
         if ($dateTime < $this->start) {
-            return $this->start;
+            /** @var DateTimeInterface $start */
+            $start = $this->start;
+
+            return $start;
         }
 
         return $this->cron->getNextRunDate($dateTime);
@@ -187,6 +193,9 @@ public static function fromJson(string $json): Frequency
         return $self;
     }
 
+    /**
+     * @return array<string, string>
+     */
     public function jsonSerialize(): array
     {
         $data = ['expression' => $this->getExpression()];
diff --git a/src/RRule.php b/src/RRule.php
index bfad0e5..3288532 100644
--- a/src/RRule.php
+++ b/src/RRule.php
@@ -158,7 +158,10 @@ public function isDue(DateTimeInterface $dateTime): bool
     public function getNextDue(DateTimeInterface $dateTime): DateTimeInterface
     {
         if ($this->isExpired($dateTime)) {
-            return $this->getEnd();
+            /** @var DateTimeInterface $end */
+            $end = $this->getEnd();
+
+            return $end;
         }
 
         $nextDue = $this->getNextRecurrences($dateTime, 1, false);
@@ -194,7 +197,9 @@ public function startAt(DateTimeInterface $start): self
         // the transformer operates only up to seconds level. See also the upstream issue #155
         $startDate->setTime($start->format('H'), $start->format('i'), $start->format('s'));
         // In case start time uses a different tz than what the rrule internally does, we force it to use the same
-        $startDate->setTimezone(new DateTimeZone($this->rrule->getTimezone()));
+        /** @var string $timeZone */
+        $timeZone = $this->rrule->getTimezone();
+        $startDate->setTimezone(new DateTimeZone($timeZone));
 
         $this->rrule->setStartDate($startDate);
 
@@ -267,7 +272,9 @@ public function getNextRecurrences(
         if (! $this->rrule->repeatsIndefinitely()) {
             // When accessing this method externally (not by using `getNextDue()`), the transformer may
             // generate recurrences beyond the configured end time.
-            $constraint = new BetweenConstraint($dateTime, $this->getEnd(), $include);
+            /** @var DateTimeInterface $end */
+            $end = $this->getEnd();
+            $constraint = new BetweenConstraint($dateTime, $end, $include);
         }
 
         // Setting the start date to a date time smaller than now causes the underlying library
@@ -284,6 +291,9 @@ public function getNextRecurrences(
         }
     }
 
+    /**
+     * @return array<string, string>
+     */
     public function jsonSerialize(): array
     {
         $data = [
@@ -323,6 +333,9 @@ public function __call(string $methodName, array $args)
             );
         }
 
-        return call_user_func_array([$this->rrule, $methodName], $args);
+        /** @var callable $callBack */
+        $callBack = [$this->rrule, $methodName];
+
+        return call_user_func_array($callBack, $args);
     }
 }
diff --git a/src/Scheduler.php b/src/Scheduler.php
index 25ad3a1..a255019 100644
--- a/src/Scheduler.php
+++ b/src/Scheduler.php
@@ -10,6 +10,7 @@
 use ipl\Scheduler\Contract\Task;
 use ipl\Stdlib\Events;
 use React\EventLoop\Loop;
+use React\EventLoop\TimerInterface;
 use React\Promise;
 use React\Promise\ExtendedPromiseInterface;
 use SplObjectStorage;
@@ -283,7 +284,9 @@ public function isValidEvent(string $event): bool
      */
     protected function cancelTask(Task $task): void
     {
-        Loop::cancelTimer($this->detachTimer($task->getUuid()));
+        /** @var TimerInterface $timer */
+        $timer = $this->detachTimer($task->getUuid());
+        Loop::cancelTimer($timer);
 
         /** @var ExtendedPromiseInterface[] $promises */
         $promises = $this->detachPromises($task->getUuid());