diff --git a/.ldrelease/config.yml b/.ldrelease/config.yml
index 3fbf486a..7604660e 100644
--- a/.ldrelease/config.yml
+++ b/.ldrelease/config.yml
@@ -8,6 +8,11 @@ publications:
   - url: https://packagist.org/packages/launchdarkly/server-sdk
     description: Packagist
 
+branches:
+  - name: main
+    description: 5.x
+  - name: 4.x
+
 jobs:
   - docker:
       image: ldcircleci/php-sdk-release:4  # Releaser's default for PHP is still php-sdk-release:3, which is PHP 7.x
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6369da04..ac3613ac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 All notable changes to the LaunchDarkly PHP SDK will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org).
 
+## [4.3.0] - 2023-01-31
+### Added:
+- Introduced support for an `application_info` config property which sets application metadata that may be used in LaunchDarkly analytics or other product features. This does not affect feature flag evaluations.
+
 ## [5.0.0] - 2023-01-04
 The latest version of this SDK supports LaunchDarkly's new custom contexts feature. Contexts are an evolution of a previously-existing concept, "users." Contexts let you create targeting rules for feature flags based on a variety of different information, including attributes pertaining to users, organizations, devices, and more. You can even combine contexts to create "multi-contexts." 
 
diff --git a/src/LaunchDarkly/Impl/Integrations/CurlEventPublisher.php b/src/LaunchDarkly/Impl/Integrations/CurlEventPublisher.php
index 179e5444..0d6a293a 100644
--- a/src/LaunchDarkly/Impl/Integrations/CurlEventPublisher.php
+++ b/src/LaunchDarkly/Impl/Integrations/CurlEventPublisher.php
@@ -4,6 +4,7 @@
 
 namespace LaunchDarkly\Impl\Integrations;
 
+use LaunchDarkly\Impl\Util;
 use LaunchDarkly\LDClient;
 use LaunchDarkly\Subsystems\EventPublisher;
 
@@ -15,7 +16,6 @@
  */
 class CurlEventPublisher implements EventPublisher
 {
-    private string $_sdkKey;
     private string $_host;
     private int $_port;
     private string $_path;
@@ -24,10 +24,11 @@ class CurlEventPublisher implements EventPublisher
     private int $_connectTimeout;
     private bool $_isWindows;
 
+    /** @var array<string, string> */
+    private array $_eventHeaders;
+
     public function __construct(string $sdkKey, array $options = [])
     {
-        $this->_sdkKey = $sdkKey;
-
         $baseUri = $options['events_uri'] ?? null;
         if (!$baseUri) {
             $baseUri = LDClient::DEFAULT_EVENTS_URI;
@@ -48,6 +49,7 @@ public function __construct(string $sdkKey, array $options = [])
             $this->_curl = $options['curl'];
         }
 
+        $this->_eventHeaders = Util::eventHeaders($sdkKey, $options['application_info'] ?? null);
         $this->_connectTimeout = $options['connect_timeout'];
         $this->_isWindows = PHP_OS_FAMILY == 'Windows';
     }
@@ -79,11 +81,15 @@ private function createCurlArgs(string $payload): string
         $scheme = $this->_ssl ? "https://" : "http://";
         $args = " -X POST";
         $args.= " --connect-timeout " . $this->_connectTimeout;
-        $args.= " -H 'Content-Type: application/json'";
-        $args.= " -H " . escapeshellarg("Authorization: " . $this->_sdkKey);
-        $args.= " -H 'User-Agent: PHPClient/" . LDClient::VERSION . "'";
-        $args.= " -H 'X-LaunchDarkly-Event-Schema: " . EventPublisher::CURRENT_SCHEMA_VERSION . "'";
-        $args.= " -H 'Accept: application/json'";
+
+        foreach ($this->_eventHeaders as $key => $value) {
+            if ($key == 'Authorization') {
+                $args.= " -H " . escapeshellarg("Authorization: " . $value);
+            } else {
+                $args.= " -H '$key: $value'";
+            }
+        }
+
         $args.= " -d " . escapeshellarg($payload);
         $args.= " " . escapeshellarg($scheme . $this->_host . ":" . $this->_port . $this->_path . "/bulk");
         return $args;
@@ -101,16 +107,8 @@ private function makeCurlRequest(string $args): bool
 
     private function createPowershellArgs(string $payloadFile): string
     {
-        $headers = [
-            'Content-Type' => 'application/json',
-            'Authorization' => $this->_sdkKey,
-            'User-Agent' => 'PHPClient/' . LDClient::VERSION,
-            'X-LaunchDarkly-Event-Schema' => EventPublisher::CURRENT_SCHEMA_VERSION,
-            'Accept' => 'application/json',
-        ];
-
         $headerString = "";
-        foreach ($headers as $key => $value) {
+        foreach ($this->_eventHeaders as $key => $value) {
             $headerString .= sprintf("'%s'='%s';", $key, $value);
         }
 
diff --git a/src/LaunchDarkly/Impl/Integrations/GuzzleEventPublisher.php b/src/LaunchDarkly/Impl/Integrations/GuzzleEventPublisher.php
index e1f6f312..2128ec84 100644
--- a/src/LaunchDarkly/Impl/Integrations/GuzzleEventPublisher.php
+++ b/src/LaunchDarkly/Impl/Integrations/GuzzleEventPublisher.php
@@ -34,13 +34,7 @@ public function __construct(string $sdkKey, array $options = [])
         $this->_eventsUri = \LaunchDarkly\Impl\Util::adjustBaseUri($baseUri);
 
         $this->_requestOptions = [
-            'headers' => [
-                'Content-Type'  => 'application/json',
-                'Authorization' => $this->_sdkKey,
-                'User-Agent'    => 'PHPClient/' . LDClient::VERSION,
-                'Accept'        => 'application/json',
-                'X-LaunchDarkly-Event-Schema' => strval(EventPublisher::CURRENT_SCHEMA_VERSION)
-            ],
+            'headers' => Util::eventHeaders($this->_sdkKey, $options['application_info'] ?? null),
             'timeout' => $options['timeout'],
             'connect_timeout' => $options['connect_timeout']
         ];
diff --git a/src/LaunchDarkly/Impl/Integrations/GuzzleFeatureRequester.php b/src/LaunchDarkly/Impl/Integrations/GuzzleFeatureRequester.php
index 6eb54f30..47d3efec 100644
--- a/src/LaunchDarkly/Impl/Integrations/GuzzleFeatureRequester.php
+++ b/src/LaunchDarkly/Impl/Integrations/GuzzleFeatureRequester.php
@@ -13,7 +13,6 @@
 use LaunchDarkly\Impl\Model\Segment;
 use LaunchDarkly\Impl\UnrecoverableHTTPStatusException;
 use LaunchDarkly\Impl\Util;
-use LaunchDarkly\LDClient;
 use LaunchDarkly\Subsystems\FeatureRequester;
 use Psr\Log\LoggerInterface;
 
@@ -48,11 +47,7 @@ public function __construct(string $baseUri, string $sdkKey, array $options)
         }
 
         $defaults = [
-            'headers' => [
-                'Authorization' => $sdkKey,
-                'Content-Type' => 'application/json',
-                'User-Agent' => 'PHPClient/' . LDClient::VERSION
-            ],
+            'headers' => Util::defaultHeaders($sdkKey, $options['application_info'] ?? null),
             'timeout' => $options['timeout'],
             'connect_timeout' => $options['connect_timeout'],
             'handler' => $stack,
diff --git a/src/LaunchDarkly/Impl/Util.php b/src/LaunchDarkly/Impl/Util.php
index 29dc720d..a0350279 100644
--- a/src/LaunchDarkly/Impl/Util.php
+++ b/src/LaunchDarkly/Impl/Util.php
@@ -6,6 +6,9 @@
 
 use DateTime;
 use DateTimeZone;
+use LaunchDarkly\LDClient;
+use LaunchDarkly\Subsystems\EventPublisher;
+use LaunchDarkly\Types\ApplicationInfo;
 use Monolog\Handler\NullHandler;
 use Monolog\Logger;
 use Psr\Log\LoggerInterface;
@@ -64,4 +67,47 @@ public static function makeNullLogger(): LoggerInterface
     {
         return new Logger('', [new NullHandler()]);
     }
+
+    /**
+     * An array of header name and values that should be used for any request
+     * made to LaunchDarkly servers.
+     *
+     * @param string $sdkKey
+     * @param ApplicationInfo|null $applicationInfo
+     * @return array<string, string>
+     */
+    public static function defaultHeaders(string $sdkKey, $applicationInfo): array
+    {
+        $headers = [
+            'Content-Type' => 'application/json',
+            'Accept' => 'application/json',
+            'Authorization' => $sdkKey,
+            'User-Agent' => 'PHPClient/' . LDClient::VERSION,
+        ];
+
+        if ($applicationInfo instanceof ApplicationInfo) {
+            $headerValue = (string) $applicationInfo;
+            if ($headerValue) {
+                $headers['X-LaunchDarkly-Tags'] = $headerValue;
+            }
+        }
+
+        return $headers;
+    }
+
+    /**
+     * An array of header name and values that should be used for any request
+     * made to the LaunchDarkly Events API.
+     *
+     * @param string $sdkKey
+     * @param ApplicationInfo|null $applicationInfo
+     * @return array
+     */
+    public static function eventHeaders(string $sdkKey, $applicationInfo): array
+    {
+        $headers = Util::defaultHeaders($sdkKey, $applicationInfo);
+        $headers['X-LaunchDarkly-Event-Schema'] = EventPublisher::CURRENT_SCHEMA_VERSION;
+
+        return $headers;
+    }
 }
diff --git a/src/LaunchDarkly/LDClient.php b/src/LaunchDarkly/LDClient.php
index ea135abe..288a2b40 100644
--- a/src/LaunchDarkly/LDClient.php
+++ b/src/LaunchDarkly/LDClient.php
@@ -16,6 +16,7 @@
 use LaunchDarkly\Impl\Util;
 use LaunchDarkly\Integrations\Guzzle;
 use LaunchDarkly\Subsystems\FeatureRequester;
+use LaunchDarkly\Types\ApplicationInfo;
 use Monolog\Handler\ErrorLogHandler;
 use Monolog\Logger;
 use Psr\Log\LoggerInterface;
@@ -75,6 +76,7 @@ class LDClient
      * Defaults to false.
      * - `private_attribute_names`: An optional array of user attribute names to be marked private. Any users sent to LaunchDarkly
      * with this configuration active will have attributes with these names removed. You can also set private attributes on a
+     * - `application_info`: An optional {@see \LaunchDarkly\Types\ApplicationInfo} instance.
      * per-user basis in the LDContext builder.
      * - Other options may be available depending on any features you are using from the `LaunchDarkly\Integrations` namespace.
      *
@@ -120,8 +122,17 @@ public function __construct(string $sdkKey, array $options = [])
             $logger = new Logger("LaunchDarkly", [new ErrorLogHandler()]);
             $options['logger'] = $logger;
         }
+
+        /** @var LoggerInterface */
         $this->_logger = $options['logger'];
 
+        $applicationInfo = $options['application_info'] ?? null;
+        if ($applicationInfo instanceof ApplicationInfo) {
+            foreach ($applicationInfo->errors() as $error) {
+                $this->_logger->warning($error);
+            }
+        }
+
         $this->_eventFactoryDefault = new EventFactory(false);
         $this->_eventFactoryWithReasons = new EventFactory(true);
 
diff --git a/src/LaunchDarkly/Types/ApplicationInfo.php b/src/LaunchDarkly/Types/ApplicationInfo.php
new file mode 100644
index 00000000..6ae77e3b
--- /dev/null
+++ b/src/LaunchDarkly/Types/ApplicationInfo.php
@@ -0,0 +1,100 @@
+<?php
+
+namespace LaunchDarkly\Types;
+
+/**
+ * An object that allows configuration of application metadata.
+ *
+ * Application metadata may be used in LaunchDarkly analytics or other product
+ * features, but does not affect feature flag evaluations.
+ *
+ * To use these properties, provide an instance of ApplicationInfo in the config
+ * parameter of the LDClient.
+ *
+ * Application values have the following restrictions:
+ * - Cannot be empty
+ * - Cannot exceed 64 characters in length
+ * - Can only contain a-z, A-Z, 0-9, period (.), dash (-), and underscore (_).
+ */
+final class ApplicationInfo
+{
+    /** @var string|null **/
+    private $id;
+
+    /** @var string|null **/
+    private $version;
+
+    /** @var array **/
+    private $errors;
+
+    public function __construct()
+    {
+        $this->id = null;
+        $this->version = null;
+        $this->errors = [];
+    }
+
+    /**
+     * Set the application id metadata identifier.
+     */
+    public function withId(string $id): ApplicationInfo
+    {
+        $this->id = $this->validateValue($id, 'id');
+
+        return $this;
+    }
+
+    /**
+     * Set the application version metadata identifier.
+     */
+    public function withVersion(string $version): ApplicationInfo
+    {
+        $this->version = $this->validateValue($version, 'version');
+
+        return $this;
+    }
+
+    /**
+     * Retrieve any validation errors that have accumulated as a result of creating this instance.
+     */
+    public function errors(): array
+    {
+        return array_values($this->errors);
+    }
+
+    public function __toString(): string
+    {
+        $parts = [];
+
+        if ($this->id !== null) {
+            $parts[] = "application-id/{$this->id}";
+        }
+
+        if ($this->version !== null) {
+            $parts[] = "application-version/{$this->version}";
+        }
+
+        return join(" ", $parts);
+    }
+
+    private function validateValue(string $value, string $label): ?string
+    {
+        $value = strval($value);
+
+        if ($value === '') {
+            return null;
+        }
+
+        if (strlen($value) > 64) {
+            $this->errors[$label] = "Application value for $label was longer than 64 characters and was discarded";
+            return null;
+        }
+
+        if (preg_match('/[^a-zA-Z0-9._-]/', $value)) {
+            $this->errors[$label] = "Application value for $label contained invalid characters and was discarded";
+            return null;
+        }
+
+        return $value;
+    }
+}
diff --git a/tests/Impl/Integrations/CurlEventPublisherTest.php b/tests/Impl/Integrations/EventPublisherTest.php
similarity index 64%
rename from tests/Impl/Integrations/CurlEventPublisherTest.php
rename to tests/Impl/Integrations/EventPublisherTest.php
index c4397be4..090a2d1c 100644
--- a/tests/Impl/Integrations/CurlEventPublisherTest.php
+++ b/tests/Impl/Integrations/EventPublisherTest.php
@@ -6,9 +6,11 @@
 use LaunchDarkly\Impl\Integrations;
 use LaunchDarkly\LDClient;
 use LaunchDarkly\Subsystems\EventPublisher;
+use LaunchDarkly\Types\ApplicationInfo;
 use PHPUnit\Framework\TestCase;
+use Psr\Log\LoggerInterface;
 
-class CurlEventPublisherTest extends TestCase
+class EventPublisherTest extends TestCase
 {
     public function setUp(): void
     {
@@ -20,16 +22,35 @@ public function setUp(): void
         $client->request('DELETE', 'http://localhost:8080/__admin/requests');
     }
 
-    public function testSendsCorrectBodyAndHeaders()
+    public function getEventPublisher(): array
+    {
+        /** @var LoggerInterface **/
+        $logger = $this->getMockBuilder(LoggerInterface::class)->getMock();
+        $appInfo = (new ApplicationInfo())->withId('my-id')->withVersion('my-version');
+
+        $config = [
+            'events_uri' => 'http://localhost:8080',
+            'timeout' => 3,
+            'connect_timeout' => 3,
+            'application_info' => $appInfo,
+            'logger' => $logger,
+        ];
+
+        $curlPublisher = new Integrations\CurlEventPublisher('sdk-key', $config);
+        $guzzlePublisher = new Integrations\GuzzleEventPublisher('sdk-key', $config);
+
+        return [
+            [$curlPublisher],
+            [$guzzlePublisher],
+        ];
+    }
+
+    /**
+     * @dataProvider getEventPublisher
+     */
+    public function testSendsCorrectBodyAndHeaders($publisher)
     {
         $event = json_encode(["key" => "user-key"]);
-        $publisher = new Integrations\CurlEventPublisher(
-            'sdk-key',
-            [
-                'events_uri' => 'http://localhost:8080',
-                'connect_timeout' => 3,
-            ]
-        );
         $publisher->publish($event);
 
         $requests = [];
@@ -67,5 +88,6 @@ public function testSendsCorrectBodyAndHeaders()
         $this->assertEquals('sdk-key', $headers['Authorization']);
         $this->assertEquals('PHPClient/' . LDClient::VERSION, $headers['User-Agent']);
         $this->assertEquals(EventPublisher::CURRENT_SCHEMA_VERSION, $headers['X-LaunchDarkly-Event-Schema']);
+        $this->assertEquals('application-id/my-id application-version/my-version', $headers['X-LaunchDarkly-Tags']);
     }
 }
diff --git a/tests/Impl/Integrations/GuzzleFeatureRequesterTest.php b/tests/Impl/Integrations/GuzzleFeatureRequesterTest.php
new file mode 100644
index 00000000..4c2badf5
--- /dev/null
+++ b/tests/Impl/Integrations/GuzzleFeatureRequesterTest.php
@@ -0,0 +1,75 @@
+<?php
+
+namespace LaunchDarkly\Tests\Impl\Integrations;
+
+use GuzzleHttp\Client;
+use LaunchDarkly\Impl\Integrations\GuzzleFeatureRequester;
+use LaunchDarkly\LDClient;
+use LaunchDarkly\Types\ApplicationInfo;
+use PHPUnit\Framework\TestCase;
+use Psr\Log\LoggerInterface;
+
+class GuzzleFeatureRequesterTest extends TestCase
+{
+    public function setUp(): void
+    {
+        if (!getenv("LD_INCLUDE_INTEGRATION_TESTS")) {
+            $this->markTestSkipped("Skipping integration test");
+        }
+
+        $client = new Client();
+        $client->request('DELETE', 'http://localhost:8080/__admin/requests');
+    }
+
+    public function testSendsCorrectHeaders(): void
+    {
+        /** @var LoggerInterface **/
+        $logger = $this->getMockBuilder(LoggerInterface::class)->getMock();
+        $appInfo = (new ApplicationInfo())->withId('my-id')->withVersion('my-version');
+
+        $config = [
+            'logger' => $logger,
+            'timeout' => 3,
+            'connect_timeout' => 3,
+            'application_info' => $appInfo,
+        ];
+
+        $requester = new GuzzleFeatureRequester('http://localhost:8080', 'sdk-key', $config);
+        $requester->getFeature("flag-key");
+
+        $requests = [];
+        $client = new Client();
+
+        // Provide time for the curl to execute
+        $start = time();
+        while (time() - $start < 5) {
+            $response = $client->request('GET', 'http://localhost:8080/__admin/requests');
+            $body = json_decode($response->getBody()->getContents(), true);
+            $requests = $body['requests'];
+
+            if ($requests) {
+                break;
+            }
+            usleep(100);
+        }
+
+        if (!$requests) {
+            $this->fail("Unable to connect to endpoint within specified timeout");
+        }
+
+        $this->assertCount(1, $requests);
+
+        $request = $requests[0]['request'];
+
+        // Validate that we hit the right endpoint
+        $this->assertEquals('/sdk/flags/flag-key', $request['url']);
+
+        // And validate that we provided all the correct headers
+        $headers = $request['headers'];
+        $this->assertEquals('application/json', $headers['Content-Type']);
+        $this->assertEquals('application/json', $headers['Accept']);
+        $this->assertEquals('sdk-key', $headers['Authorization']);
+        $this->assertEquals('PHPClient/' . LDClient::VERSION, $headers['User-Agent']);
+        $this->assertEquals('application-id/my-id application-version/my-version', $headers['X-LaunchDarkly-Tags']);
+    }
+}
diff --git a/tests/Types/ApplicationInfoTest.php b/tests/Types/ApplicationInfoTest.php
new file mode 100644
index 00000000..9f232552
--- /dev/null
+++ b/tests/Types/ApplicationInfoTest.php
@@ -0,0 +1,79 @@
+<?php
+
+namespace LaunchDarkly\Tests\Types;
+
+use LaunchDarkly\Types\ApplicationInfo;
+use PHPUnit\Framework\TestCase;
+
+class ApplicationInfoTest extends TestCase
+{
+    /** @var ApplicationInfo */
+    private $appInfo;
+
+    public function setUp(): void
+    {
+        $this->appInfo = new ApplicationInfo();
+    }
+
+    public function testNewInstanceIsEmpty(): void
+    {
+        $this->assertEquals((string) $this->appInfo, "", "Empty app info isn't empty!");
+    }
+
+    public function testCanSetValuesAsExpected(): void
+    {
+        $this->appInfo
+             ->withId("my-id")
+             ->withVersion("my-version");
+
+        $this->assertEquals("application-id/my-id application-version/my-version", (string) $this->appInfo, "Failed to set id and version correctly");
+        $this->assertEmpty($this->appInfo->errors());
+    }
+
+    public function testIgnoresEmptyValues(): void
+    {
+        $this->appInfo->withId("")->withVersion("");
+
+        $this->assertEquals("", (string) $this->appInfo, "Failed to set id and version correctly");
+        $this->assertEquals([], $this->appInfo->errors(), "Failed to set id and version correctly");
+    }
+
+    /**
+     * @return array<int,array<int,string>>
+     */
+    public function invalidValues(): array
+    {
+        return [
+            [' ', 'Application value for %s contained invalid characters and was discarded'],
+            [' ', 'Application value for %s contained invalid characters and was discarded'],
+            ['@', 'Application value for %s contained invalid characters and was discarded'],
+            ['@', 'Application value for %s contained invalid characters and was discarded'],
+            ['abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.-_a', 'Application value for %s was longer than 64 characters and was discarded'], // Too long
+        ];
+    }
+
+    /**
+     * @dataProvider invalidValues
+     */
+    public function testIgnoresInvalidValuesAndLogsAppropriately(string $value, string $error): void
+    {
+        $this->appInfo->withId($value)->withVersion($value);
+
+        $errors = [
+            sprintf($error, 'id'),
+            sprintf($error, 'version'),
+        ];
+
+        $this->assertEquals("", (string) $this->appInfo, "Failed to set id and version correctly");
+        $this->assertEquals($errors, $this->appInfo->errors(), "Failed to set id and version correctly");
+    }
+
+    public function testOnlyTracksMostRecentFailure(): void
+    {
+        $this->appInfo->withId('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.-_a');
+        $this->assertEquals(["Application value for id was longer than 64 characters and was discarded"], $this->appInfo->errors(), "Most recent error wasn't retained");
+
+        $this->appInfo->withId('@');
+        $this->assertEquals(["Application value for id contained invalid characters and was discarded"], $this->appInfo->errors(), "Most recent error wasn't retained");
+    }
+}