diff --git a/src/Zipkin/ErrorParser.php b/src/Zipkin/ErrorParser.php
index d08b67d..8dc22f3 100644
--- a/src/Zipkin/ErrorParser.php
+++ b/src/Zipkin/ErrorParser.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Zipkin;
 
 use Zipkin\Recording\Span;
@@ -12,5 +14,11 @@
  */
 interface ErrorParser
 {
+    /**
+     * Parses the error into a set of tags.
+     *
+     * @param Throwable $e
+     * @return array<string,string>
+     */
     public function parseTags(Throwable $e): array;
 }
diff --git a/src/Zipkin/Instrumentation/Http/Client/HttpClientTracing.php b/src/Zipkin/Instrumentation/Http/Client/HttpClientTracing.php
index f3bb1b8..8775f2a 100644
--- a/src/Zipkin/Instrumentation/Http/Client/HttpClientTracing.php
+++ b/src/Zipkin/Instrumentation/Http/Client/HttpClientTracing.php
@@ -27,8 +27,8 @@ class HttpClientTracing
 
     public function __construct(
         Tracing $tracing,
-        HttpClientParser $parser = null,
-        callable $requestSampler = null
+        ?HttpClientParser $parser = null,
+        ?callable $requestSampler = null
     ) {
         $this->tracing = $tracing;
         $this->parser = $parser ?? new DefaultHttpClientParser();
diff --git a/src/Zipkin/Instrumentation/Http/Server/HttpServerTracing.php b/src/Zipkin/Instrumentation/Http/Server/HttpServerTracing.php
index 53b7395..abc7c2e 100644
--- a/src/Zipkin/Instrumentation/Http/Server/HttpServerTracing.php
+++ b/src/Zipkin/Instrumentation/Http/Server/HttpServerTracing.php
@@ -26,8 +26,8 @@ class HttpServerTracing
 
     public function __construct(
         Tracing $tracing,
-        HttpServerParser $parser = null,
-        callable $requestSampler = null
+        ?HttpServerParser $parser = null,
+        ?callable $requestSampler = null
     ) {
         $this->tracing = $tracing;
         $this->parser = $parser ?? new DefaultHttpServerParser();
diff --git a/src/Zipkin/Instrumentation/Mysqli/Mysqli.php b/src/Zipkin/Instrumentation/Mysqli/Mysqli.php
index e25011b..95f3baf 100644
--- a/src/Zipkin/Instrumentation/Mysqli/Mysqli.php
+++ b/src/Zipkin/Instrumentation/Mysqli/Mysqli.php
@@ -6,8 +6,8 @@
 use const Zipkin\Tags\ERROR;
 use Zipkin\Tracer;
 use Zipkin\Span;
-
 use Zipkin\Endpoint;
+use Zipkin\Kind;
 use InvalidArgumentException;
 
 /**
@@ -24,113 +24,105 @@ final class Mysqli extends \Mysqli
     ];
 
     private Tracer $tracer;
-
     private array $options;
 
     public function __construct(
         Tracer $tracer,
         array $options = [],
-        string $host = null,
-        string $user = null,
-        string $password = null,
+        ?string $host = null,
+        ?string $user = null,
+        ?string $password = null,
         string $database = '',
-        int $port = null,
-        string $socket = null
+        ?int $port = null,
+        ?string $socket = null
     ) {
         self::validateOptions($options);
         $this->tracer = $tracer;
         $this->options = $options + self::DEFAULT_OPTIONS;
+
+        $defaultHost = \ini_get('mysqli.default_host') ?: '';
+        $defaultUser = \ini_get('mysqli.default_user') ?: '';
+        $defaultPassword = \ini_get('mysqli.default_pw') ?: '';
+        $defaultPort = (int) (\ini_get('mysqli.default_port') ?: 3306);
+        $defaultSocket = \ini_get('mysqli.default_socket') ?: '';
+
         parent::__construct(
-            $host ?? (ini_get('mysqli.default_host') ?: ''),
-            $user ?? (ini_get('mysqli.default_user') ?: ''),
-            $password ?? (ini_get('mysqli.default_pw') ?: ''),
+            $host ?? $defaultHost,
+            $user ?? $defaultUser,
+            $password ?? $defaultPassword,
             $database,
-            $port ?? (($defaultPort = ini_get('mysqli.default_port')) ? (int) $defaultPort : 3306),
-            $socket ?? (ini_get('mysqli.default_socket') ?: '')
+            $port ?? $defaultPort,
+            $socket ?? $defaultSocket
         );
     }
 
-    private static function validateOptions(array $opts): void
+    /**
+     * {@inheritdoc}
+     */
+    public function query(string $query, int $result_mode = MYSQLI_STORE_RESULT): mysqli_result|bool
     {
-        if (array_key_exists('tag_query', $opts) && ($opts['tag_query'] !== (bool) $opts['tag_query'])) {
-            throw new InvalidArgumentException('Invalid tag_query, bool expected');
-        }
-
-        if (array_key_exists('remote_endpoint', $opts) && !($opts['remote_endpoint'] instanceof Endpoint)) {
-            throw new InvalidArgumentException(sprintf('Invalid remote_endpoint, %s expected', Endpoint::class));
-        }
-
-        if (array_key_exists('default_tags', $opts) && ($opts['default_tags'] !== (array) $opts['default_tags'])) {
-            throw new InvalidArgumentException('Invalid default_tags, array expected');
-        }
-    }
+        $span = $this->tracer->nextSpan();
+        $span->start();
+        $span->setName('query');
+        $span->setKind(Kind\CLIENT);
 
-    private function addsTagsAndRemoteEndpoint(Span $span, string $query = null): void
-    {
-        if ($query !== null && $this->options['tag_query']) {
+        if ($this->options['tag_query']) {
             $span->tag('sql.query', $query);
         }
 
-        if ($this->options['remote_endpoint'] !== null) {
-            $span->setRemoteEndpoint($this->options['remote_endpoint']);
-        }
-
         foreach ($this->options['default_tags'] as $key => $value) {
             $span->tag($key, $value);
         }
-    }
-
-    /**
-     * Performs a query on the database
-     *
-     * @return mysqli_result|bool
-     * @alias mysqli_query
-     */
-    public function query(string $query, int $resultmode = MYSQLI_STORE_RESULT)
-    {
-        if ($resultmode === MYSQLI_ASYNC) {
-            // if $resultmode is async, making the timing on this execution
-            // does not make much sense. For now we just skip tracing on this.
-            return parent::query($query, $resultmode);
-        }
 
-        $span = $this->tracer->nextSpan();
-        $span->setName('sql/query');
-        $this->addsTagsAndRemoteEndpoint($span, $query);
-        if ($this->options['tag_query']) {
-            $span->tag('sql.query', $query);
+        if ($this->options['remote_endpoint'] !== null) {
+            $span->setRemoteEndpoint($this->options['remote_endpoint']);
         }
 
-        $span->start();
         try {
-            $result = parent::query($query, $resultmode);
+            $result = parent::query($query, $result_mode);
             if ($result === false) {
-                $span->tag(ERROR, 'true');
+                $span->tag(ERROR, $this->error);
             }
             return $result;
+        } catch (\Throwable $e) {
+            $span->tag(ERROR, $e->getMessage());
+            throw $e;
         } finally {
             $span->finish();
         }
     }
 
     /**
-     * @return bool
-     * @alias mysqli_real_query
+     * {@inheritdoc}
      */
-    // phpcs:ignore PSR1.Methods.CamelCapsMethodName
-    public function real_query(string $query)
+    public function real_query(string $query): bool
     {
         $span = $this->tracer->nextSpan();
-        $span->setName('sql/query');
-        $this->addsTagsAndRemoteEndpoint($span, $query);
-
         $span->start();
+        $span->setName('real_query');
+        $span->setKind(Kind\CLIENT);
+
+        if ($this->options['tag_query']) {
+            $span->tag('sql.query', $query);
+        }
+
+        foreach ($this->options['default_tags'] as $key => $value) {
+            $span->tag($key, $value);
+        }
+
+        if ($this->options['remote_endpoint'] !== null) {
+            $span->setRemoteEndpoint($this->options['remote_endpoint']);
+        }
+
         try {
             $result = parent::real_query($query);
             if ($result === false) {
-                $span->tag(ERROR, 'true');
+                $span->tag(ERROR, $this->error);
             }
             return $result;
+        } catch (\Throwable $e) {
+            $span->tag(ERROR, $e->getMessage());
+            throw $e;
         } finally {
             $span->finish();
         }
@@ -138,58 +130,63 @@ public function real_query(string $query)
 
     /**
      * {@inheritdoc}
-     * @alias mysqli_begin_transaction
      */
-    // phpcs:ignore PSR1.Methods.CamelCapsMethodName
-    public function begin_transaction($flags = 0, $name = null)
+    public function begin_transaction(int $flags = 0, ?string $name = null): bool
     {
         $span = $this->tracer->nextSpan();
-        $span->setName('sql/begin_transaction');
-        $this->addsTagsAndRemoteEndpoint($span);
         $span->start();
-        if ($name !== null) {
-            $span->tag('mysqli.transaction_name', (string) $name);
+        $span->setName('begin_transaction');
+        $span->setKind(Kind\CLIENT);
+
+        foreach ($this->options['default_tags'] as $key => $value) {
+            $span->tag($key, $value);
         }
-        try {
-            if ($name === null) {
-                $result = parent::begin_transaction($flags);
-            } else {
-                $result = parent::begin_transaction($flags, $name);
-            }
 
+        if ($this->options['remote_endpoint'] !== null) {
+            $span->setRemoteEndpoint($this->options['remote_endpoint']);
+        }
+
+        try {
+            $result = parent::begin_transaction($flags, $name);
             if ($result === false) {
-                $span->tag(ERROR, 'true');
+                $span->tag(ERROR, $this->error);
             }
             return $result;
+        } catch (\Throwable $e) {
+            $span->tag(ERROR, $e->getMessage());
+            throw $e;
         } finally {
             $span->finish();
         }
     }
 
     /**
-     * @return bool
-     * @alias mysqli_commit
+     * {@inheritdoc}
      */
-    public function commit(int $flags = -1, ?string $name = null)
+    public function commit(int $flags = 0, ?string $name = null): bool
     {
         $span = $this->tracer->nextSpan();
-        $span->setName('sql/begin_transaction');
-        $this->addsTagsAndRemoteEndpoint($span);
         $span->start();
-        if ($name !== null) {
-            $span->tag('mysqli.transaction_name', $name);
+        $span->setName('commit');
+        $span->setKind(Kind\CLIENT);
+
+        foreach ($this->options['default_tags'] as $key => $value) {
+            $span->tag($key, $value);
         }
-        try {
-            if ($name === null) {
-                $result = parent::commit($flags);
-            } else {
-                $result = parent::commit($flags, $name);
-            }
 
+        if ($this->options['remote_endpoint'] !== null) {
+            $span->setRemoteEndpoint($this->options['remote_endpoint']);
+        }
+
+        try {
+            $result = parent::commit($flags, $name);
             if ($result === false) {
-                $span->tag(ERROR, 'true');
+                $span->tag(ERROR, $this->error);
             }
             return $result;
+        } catch (\Throwable $e) {
+            $span->tag(ERROR, $e->getMessage());
+            throw $e;
         } finally {
             $span->finish();
         }
@@ -197,29 +194,48 @@ public function commit(int $flags = -1, ?string $name = null)
 
     /**
      * {@inheritdoc}
-     * @alias mysqli_rollback
      */
-    public function rollback($flags = 0, $name = null)
+    public function rollback(int $flags = 0, ?string $name = null): bool
     {
         $span = $this->tracer->nextSpan();
-        $span->setName('sql/rollback');
-        $this->addsTagsAndRemoteEndpoint($span);
         $span->start();
-        if ($name !== null) {
-            $span->tag('mysqli.transaction_name', (string) $name);
+        $span->setName('rollback');
+        $span->setKind(Kind\CLIENT);
+
+        foreach ($this->options['default_tags'] as $key => $value) {
+            $span->tag($key, $value);
         }
+
+        if ($this->options['remote_endpoint'] !== null) {
+            $span->setRemoteEndpoint($this->options['remote_endpoint']);
+        }
+
         try {
-            if ($name === null) {
-                $result = parent::commit($flags);
-            } else {
-                $result = parent::commit($flags, $name);
-            }
+            $result = parent::rollback($flags, $name);
             if ($result === false) {
-                $span->tag(ERROR, 'true');
+                $span->tag(ERROR, $this->error);
             }
             return $result;
+        } catch (\Throwable $e) {
+            $span->tag(ERROR, $e->getMessage());
+            throw $e;
         } finally {
             $span->finish();
         }
     }
+
+    private static function validateOptions(array $opts): void
+    {
+        if (isset($opts['remote_endpoint']) && !$opts['remote_endpoint'] instanceof Endpoint) {
+            throw new InvalidArgumentException(
+                \sprintf('Invalid remote_endpoint. Expected Endpoint, got %s', \gettype($opts['remote_endpoint']))
+            );
+        }
+
+        if (isset($opts['default_tags']) && !\is_array($opts['default_tags'])) {
+            throw new InvalidArgumentException(
+                \sprintf('Invalid default_tags. Expected array, got %s', \gettype($opts['default_tags']))
+            );
+        }
+    }
 }
diff --git a/src/Zipkin/Kind.php b/src/Zipkin/Kind.php
index fda3b9b..e5dcfc9 100644
--- a/src/Zipkin/Kind.php
+++ b/src/Zipkin/Kind.php
@@ -28,3 +28,11 @@
 * #PRODUCER} of this message is the {@link TraceContext#parentId()} of this span.
 */
 const CONSUMER = 'CONSUMER';
+
+
+class Kind {
+    const CLIENT = 'CLIENT';
+    const SERVER = 'SERVER';
+    const PRODUCER = 'PRODUCER';
+    const CONSUMER = 'CONSUMER';
+}
diff --git a/src/Zipkin/NoopSpan.php b/src/Zipkin/NoopSpan.php
index be148f1..b8a78f2 100644
--- a/src/Zipkin/NoopSpan.php
+++ b/src/Zipkin/NoopSpan.php
@@ -37,7 +37,7 @@ public function getContext(): TraceContext
      * Spans can be modified before calling start. For example, you can add tags to the span and
      * set its name without lock contention.
      */
-    public function start(int $timestamp = null): void
+    public function start(?int $timestamp = null): void
     {
     }
 
@@ -87,7 +87,7 @@ public function setError(Throwable $e): void
      * @return void
      * @see Annotations
      */
-    public function annotate(string $value, int $timestamp = null): void
+    public function annotate(string $value, ?int $timestamp = null): void
     {
     }
 
@@ -113,7 +113,7 @@ public function abandon(): void
      * {@link zipkin.Span#duration Zipkin's span duration} is derived by subtracting the start
      * timestamp from this, and set when appropriate.
      */
-    public function finish(int $timestamp = null): void
+    public function finish(?int $timestamp = null): void
     {
     }
 
diff --git a/src/Zipkin/NoopSpanCustomizer.php b/src/Zipkin/NoopSpanCustomizer.php
index 49d74d9..fffb64a 100644
--- a/src/Zipkin/NoopSpanCustomizer.php
+++ b/src/Zipkin/NoopSpanCustomizer.php
@@ -23,7 +23,7 @@ public function tag(string $key, string $value): void
     /**
      * {@inheritdoc}
      */
-    public function annotate(string $value, int $timestamp = null): void
+    public function annotate(string $value, ?int $timestamp = null): void
     {
     }
 }
diff --git a/src/Zipkin/Propagation/B3.php b/src/Zipkin/Propagation/B3.php
index 73cc8b5..274d95f 100644
--- a/src/Zipkin/Propagation/B3.php
+++ b/src/Zipkin/Propagation/B3.php
@@ -147,7 +147,7 @@ final class B3 implements Propagation
      * same injector list.
      */
     public function __construct(
-        LoggerInterface $logger = null,
+        ?LoggerInterface $logger = null,
         array $kindInjectors = []
     ) {
         $this->logger = $logger ?: new NullLogger();
diff --git a/src/Zipkin/Propagation/CurrentTraceContext.php b/src/Zipkin/Propagation/CurrentTraceContext.php
index 7b5a548..5d8bafb 100644
--- a/src/Zipkin/Propagation/CurrentTraceContext.php
+++ b/src/Zipkin/Propagation/CurrentTraceContext.php
@@ -18,9 +18,12 @@
 
 final class CurrentTraceContext
 {
+    /**
+     * @var ?TraceContext
+     */
     private ?TraceContext $context;
 
-    public function __construct(TraceContext $currentContext = null)
+    public function __construct(?TraceContext $currentContext = null)
     {
         $this->context = $currentContext;
     }
@@ -40,7 +43,7 @@ public function getContext(): ?TraceContext
      * @param TraceContext|null $currentContext
      * @return callable():void The scope closed
      */
-    public function createScopeAndRetrieveItsCloser(TraceContext $currentContext = null): callable
+    public function createScopeAndRetrieveItsCloser(?TraceContext $currentContext = null): callable
     {
         $previous = $this->context;
         $self = $this;
diff --git a/src/Zipkin/RealSpan.php b/src/Zipkin/RealSpan.php
index 33b7eaf..cc8ea07 100644
--- a/src/Zipkin/RealSpan.php
+++ b/src/Zipkin/RealSpan.php
@@ -48,11 +48,11 @@ public function getContext(): TraceContext
      * Spans can be modified before calling start. For example, you can add tags to the span and
      * set its name without lock contention.
      *
-     * @param int $timestamp
+     * @param int|null $timestamp
      * @return void
      * @throws \InvalidArgumentException
      */
-    public function start(int $timestamp = null): void
+    public function start(?int $timestamp = null): void
     {
         if ($timestamp !== null && !isValid($timestamp)) {
             throw new InvalidArgumentException(
@@ -117,7 +117,7 @@ public function setError(Throwable $e): void
      * @throws \InvalidArgumentException
      * @see Zipkin\Annotations
      */
-    public function annotate(string $value, int $timestamp = null): void
+    public function annotate(string $value, ?int $timestamp = null): void
     {
         if ($timestamp !== null && !isValid($timestamp)) {
             throw new InvalidArgumentException(
@@ -161,7 +161,7 @@ public function abandon(): void
      * @return void
      * @throws \InvalidArgumentException
      */
-    public function finish(int $timestamp = null): void
+    public function finish(?int $timestamp = null): void
     {
         if ($timestamp !== null && !isValid($timestamp)) {
             throw new InvalidArgumentException('Invalid timestamp');
diff --git a/src/Zipkin/Recording/Span.php b/src/Zipkin/Recording/Span.php
index 82c68d4..6d25e05 100644
--- a/src/Zipkin/Recording/Span.php
+++ b/src/Zipkin/Recording/Span.php
@@ -262,13 +262,13 @@ public function setRemoteEndpoint(Endpoint $remoteEndpoint): void
     }
 
     /**
-     * Completes and reports the span. If no finish timestamp is specified
-     * we don't compute the duration but the span is still reporterd. This
-     * usually happens when a span is flushed manually.
+     * Records the final timestamp, calculates duration and puts the span
+     * in the recorder.
      *
-     * @param int|null $finishTimestamp
+     * @param int|null $finishTimestamp When null, the value of {@link Timestamp\now()}
+     * is used.
      */
-    public function finish(int $finishTimestamp = null): void
+    public function finish(?int $finishTimestamp = null): void
     {
         if ($this->finished) {
             return;
diff --git a/src/Zipkin/Reporter.php b/src/Zipkin/Reporter.php
index 983982e..9d43b90 100644
--- a/src/Zipkin/Reporter.php
+++ b/src/Zipkin/Reporter.php
@@ -9,7 +9,9 @@
 interface Reporter
 {
     /**
-     * @param MutableSpan[] $spans
+     * Sends the given spans to the transport.
+     *
+     * @param array<MutableSpan> $spans
      * @return void
      */
     public function report(array $spans): void;
diff --git a/src/Zipkin/Reporters/Http.php b/src/Zipkin/Reporters/Http.php
index 14f9cdc..ee20ca7 100644
--- a/src/Zipkin/Reporters/Http.php
+++ b/src/Zipkin/Reporters/Http.php
@@ -11,8 +11,8 @@
 use Zipkin\Reporter;
 use Zipkin\Recording\ReadbackSpan;
 use RuntimeException;
-use Psr\Log\NullLogger;
 use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
 
 final class Http implements Reporter
 {
@@ -51,9 +51,9 @@ final class Http implements Reporter
      */
     public function __construct(
         array $options = [],
-        ClientFactory $requesterFactory = null,
-        LoggerInterface $logger = null,
-        SpanSerializer $serializer = null
+        ?ClientFactory $requesterFactory = null,
+        ?LoggerInterface $logger = null,
+        ?SpanSerializer $serializer = null
     ) {
         $this->options = \array_merge(self::DEFAULT_OPTIONS, $options);
         $this->clientFactory = $requesterFactory ?? CurlFactory::create();
diff --git a/src/Zipkin/Reporters/JsonV2Serializer.php b/src/Zipkin/Reporters/JsonV2Serializer.php
index c7a05e1..c3d9894 100644
--- a/src/Zipkin/Reporters/JsonV2Serializer.php
+++ b/src/Zipkin/Reporters/JsonV2Serializer.php
@@ -13,7 +13,7 @@ class JsonV2Serializer implements SpanSerializer
 {
     private ErrorParser $errorParser;
 
-    public function __construct(ErrorParser $errorParser = null)
+    public function __construct(?ErrorParser $errorParser = null)
     {
         $this->errorParser = $errorParser ?? new DefaultErrorParser();
     }
diff --git a/src/Zipkin/Reporters/Log.php b/src/Zipkin/Reporters/Log.php
index d2cebd1..e9eae5b 100644
--- a/src/Zipkin/Reporters/Log.php
+++ b/src/Zipkin/Reporters/Log.php
@@ -16,7 +16,7 @@ final class Log implements Reporter
 
     public function __construct(
         LoggerInterface $logger,
-        SpanSerializer $serializer = null
+        ?SpanSerializer $serializer = null
     ) {
         $this->logger = $logger;
         $this->serializer = $serializer ?? new JsonV2Serializer();
diff --git a/src/Zipkin/Span.php b/src/Zipkin/Span.php
index d8fcdac..fcb3cb3 100644
--- a/src/Zipkin/Span.php
+++ b/src/Zipkin/Span.php
@@ -14,9 +14,14 @@ interface Span
      * When true, no recording is done and nothing is reported to zipkin. However, this span should
      * still be injected into outgoing requests. Use this flag to avoid performing expensive
      * computation.
+     *
+     * @return bool
      */
     public function isNoop(): bool;
 
+    /**
+     * @return TraceContext
+     */
     public function getContext(): TraceContext;
 
     /**
@@ -24,11 +29,18 @@ public function getContext(): TraceContext;
      *
      * Spans can be modified before calling start. For example, you can add tags to the span and
      * set its name without lock contention.
+     *
+     * @param int|null $timestamp
+     * @return void
+     * @throws \InvalidArgumentException
      */
-    public function start(int $timestamp = null): void;
+    public function start(?int $timestamp = null): void;
 
     /**
      * Sets the string name for the logical operation this span represents.
+     *
+     * @param string $name
+     * @return void
      */
     public function setName(string $name): void;
 
@@ -38,16 +50,20 @@ public function setName(string $name): void;
      * and that plus its duration as "ss".
      *
      * The value must be strictly one of the ones listed in {@link Kind}.
+     *
+     * @param string $kind one of Kind\CLIENT, Kind\SERVER, Kind\CONSUMER and
+     * Kind\PRODUCER
+     * @return void
      */
     public function setKind(string $kind): void;
 
     /**
      * Tags give your span context for search, viewing and analysis. For example, a key
-     * "your_app.version" would let you lookup spans by version. A tag {@link Zipkin\Tags\SQL_QUERY}
-     * isn't searchable, but it can help in debugging when viewing a trace.
+     * "your_app.version" would let you lookup spans by version. A tag "sql.query" isn't searchable,
+     * but might show in the span view for a database call.
      *
      * @param string $key Name used to lookup spans, such as "your_app.version". See {@link Zipkin\Tags} for
-     * standard ones.
+     * common ones.
      * @param string $value
      * @return void
      */
@@ -65,9 +81,9 @@ public function setError(Throwable $e): void;
      * @param string $value A short tag indicating the event, like "finagle.retry"
      * @param int|null $timestamp
      * @return void
-     * @see Annotations
+     * @throws \InvalidArgumentException
      */
-    public function annotate(string $value, int $timestamp = null): void;
+    public function annotate(string $value, ?int $timestamp = null): void;
 
     /**
      * For a client span, this would be the server's address.
@@ -94,17 +110,18 @@ public function abandon(): void;
      *
      * @param int|null $timestamp
      * @return void
+     * @throws \InvalidArgumentException
      */
-    public function finish(int $timestamp = null): void;
+    public function finish(?int $timestamp = null): void;
 
     /**
      * Reports the span, even if unfinished. Most users will not call this method.
      *
      * This primarily supports two use cases: one-way spans and orphaned spans.
-     * For example, a one-way span can be modeled as a span where one tracer calls start and another
+     * For example, a one-way span can be modeled as a span where one flusher calls start and another
      * calls finish. In order to report that span from its origin, flush must be called.
      *
-     * Another example is where a user did not call finish within a deadline or before a shutdown
+     * Another example is where a user didn't call finish within a deadline or before a shutdown
      * occurs. By flushing, you can report what was in progress.
      *
      * @return void
diff --git a/src/Zipkin/SpanCustomizer.php b/src/Zipkin/SpanCustomizer.php
index 5a56459..918505b 100644
--- a/src/Zipkin/SpanCustomizer.php
+++ b/src/Zipkin/SpanCustomizer.php
@@ -41,5 +41,5 @@ public function tag(string $key, string $value): void;
      * @return void
      * @see Annotations
      */
-    public function annotate(string $value, int $timestamp = null): void;
+    public function annotate(string $value, ?int $timestamp = null): void;
 }
diff --git a/src/Zipkin/SpanCustomizerShield.php b/src/Zipkin/SpanCustomizerShield.php
index 06e94df..ca60c95 100644
--- a/src/Zipkin/SpanCustomizerShield.php
+++ b/src/Zipkin/SpanCustomizerShield.php
@@ -37,7 +37,7 @@ public function tag(string $key, string $value): void
     /**
      * {@inheritdoc}
      */
-    public function annotate(string $value, int $timestamp = null): void
+    public function annotate(string $value, ?int $timestamp = null): void
     {
         $this->delegate->annotate($value, $timestamp);
     }
diff --git a/src/Zipkin/Tracer.php b/src/Zipkin/Tracer.php
index 4d40b81..42ddfa7 100644
--- a/src/Zipkin/Tracer.php
+++ b/src/Zipkin/Tracer.php
@@ -70,10 +70,10 @@ public function __construct(
      * }
      * }</pre>
      *
-     * @param SamplingFlags $samplingFlags
-     * @return Span
+     * @param SamplingFlags|null $samplingFlags the sampling flags, defaults to SampleFlags::EMPTY
+     * @return Span a span that has not been started
      */
-    public function newTrace(SamplingFlags $samplingFlags = null): Span
+    public function newTrace(?SamplingFlags $samplingFlags = null): Span
     {
         if ($samplingFlags === null) {
             $samplingFlags = DefaultSamplingFlags::createAsEmpty();
@@ -135,11 +135,11 @@ public function joinSpan(TraceContext $context): Span
      * finish the span. Not only is it safe to call the closer, you must call the closer to end the scope, or
      * risk leaking resources associated with the scope.
      *
-     * @param Span $span to place into scope or null to clear the scope
+     * @param Span|null $span to place into scope or null to clear the scope
      *
      * @return callable():void The scope closer
      */
-    public function openScope(Span $span = null): callable
+    public function openScope(?Span $span = null): callable
     {
         return $this->currentTraceContext->createScopeAndRetrieveItsCloser(
             $span === null ? null : $span->getContext()
@@ -189,11 +189,12 @@ public function flush(): void
      * whatever the {@link #currentSpan()} was. Make sure you re-apply {@link #withSpanInScope(Span)}
      * so that data is written to the correct trace.
      *
-     * @param SamplingFlags|TraceContext $contextOrFlags
+     * @param TraceContext|SamplingFlags|null $contextOrFlags Either the trace context
+     * to use to create the new span or the SamplingFlags
      * @return Span
      * @throws \RuntimeException
      */
-    public function nextSpan(SamplingFlags $contextOrFlags = null): Span
+    public function nextSpan(?SamplingFlags $contextOrFlags = null): Span
     {
         if ($contextOrFlags === null) {
             $parent = $this->currentTraceContext->getContext();
@@ -230,7 +231,7 @@ public function nextSpan(SamplingFlags $contextOrFlags = null): Span
     public function nextSpanWithSampler(
         callable $sampler,
         array $args = [],
-        SamplingFlags $contextOrFlags = null
+        ?SamplingFlags $contextOrFlags = null
     ): Span {
         if ($contextOrFlags === null) {
             $contextOrFlags = $this->currentTraceContext->getContext();