diff --git a/composer.json b/composer.json
index 50fbe1cc..836c9dfa 100644
--- a/composer.json
+++ b/composer.json
@@ -17,7 +17,8 @@
         "php": ">=8.1",
         "monolog/monolog": "^2.0|^3.0",
         "psr/cache": "^3.0",
-        "psr/log": "^1.0|^2.0|^3.0"
+        "psr/log": "^1.0|^2.0|^3.0",
+        "ramsey/uuid": "^4.7"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^3.15.0",
diff --git a/src/LaunchDarkly/Impl/Util.php b/src/LaunchDarkly/Impl/Util.php
index 2f73ab51..05649307 100644
--- a/src/LaunchDarkly/Impl/Util.php
+++ b/src/LaunchDarkly/Impl/Util.php
@@ -120,6 +120,10 @@ public static function defaultHeaders(string $sdkKey, array $options): array
             }
         }
 
+        if (!empty($options['instance_id'])) {
+            $headers['X-LaunchDarkly-Instance-Id'] = $options['instance_id'];
+        }
+
         return $headers;
     }
 
diff --git a/src/LaunchDarkly/LDClient.php b/src/LaunchDarkly/LDClient.php
index e8271b99..900cc1b3 100644
--- a/src/LaunchDarkly/LDClient.php
+++ b/src/LaunchDarkly/LDClient.php
@@ -25,6 +25,7 @@
 use Monolog\Handler\ErrorLogHandler;
 use Monolog\Logger;
 use Psr\Log\LoggerInterface;
+use Ramsey\Uuid\Uuid;
 
 /**
  * A client for the LaunchDarkly API.
@@ -92,6 +93,17 @@ class LDClient
      */
     public function __construct(string $sdkKey, array $options = [])
     {
+        $apcuLoaded = extension_loaded('apcu');
+
+        if ($apcuLoaded && \apcu_enabled()) {
+            $options['instance_id'] = \apcu_entry('ld::instanceid', function ($key) {
+                $uuid = Uuid::uuid4()->toString();
+                return $uuid;
+            });
+        } elseif (php_sapi_name() === 'cli') {
+            $options['instance_id'] = Uuid::uuid4()->toString();
+        }
+
         $this->_sdkKey = $sdkKey;
         if (!isset($options['base_uri'])) {
             $this->_baseUri = self::DEFAULT_BASE_URI;
diff --git a/test-service/TestService.php b/test-service/TestService.php
index 7b4cee7b..8b99e172 100644
--- a/test-service/TestService.php
+++ b/test-service/TestService.php
@@ -77,6 +77,7 @@ public function getStatus(): array
                 'migrations',
                 'event-sampling',
                 'inline-context-all',
+                'instance-id',
                 'anonymous-redaction',
                 'client-prereq-events',
                 'big-segments'
diff --git a/tests/Impl/Integrations/EventPublisherTest.php b/tests/Impl/Integrations/EventPublisherTest.php
index 3d68f21c..d5539926 100644
--- a/tests/Impl/Integrations/EventPublisherTest.php
+++ b/tests/Impl/Integrations/EventPublisherTest.php
@@ -34,6 +34,7 @@ public function getEventPublisher(): array
             'connect_timeout' => 3,
             'application_info' => $appInfo,
             'logger' => $logger,
+            'instance_id' => 'my-instance-id',
         ];
 
         $curlPublisher = new Integrations\CurlEventPublisher('sdk-key', $config);
@@ -90,5 +91,6 @@ public function testSendsCorrectBodyAndHeaders($publisher)
         $this->assertEquals(EventPublisher::CURRENT_SCHEMA_VERSION, $headers['X-LaunchDarkly-Event-Schema']);
         $this->assertArrayHasKey('X-LaunchDarkly-Unsummarized', $headers);
         $this->assertEquals('application-id/my-id application-version/my-version', $headers['X-LaunchDarkly-Tags']);
+        $this->assertEquals('my-instance-id', $headers['X-LaunchDarkly-Instance-Id']);
     }
 }
diff --git a/tests/Impl/Integrations/GuzzleFeatureRequesterTest.php b/tests/Impl/Integrations/GuzzleFeatureRequesterTest.php
index ec74046a..3dc323b3 100644
--- a/tests/Impl/Integrations/GuzzleFeatureRequesterTest.php
+++ b/tests/Impl/Integrations/GuzzleFeatureRequesterTest.php
@@ -32,6 +32,7 @@ public function testSendsCorrectHeaders(): void
             'timeout' => 3,
             'connect_timeout' => 3,
             'application_info' => $appInfo,
+            'instance_id' => 'my-instance-id',
         ];
 
         $requester = new GuzzleFeatureRequester('http://localhost:8080', 'sdk-key', $config);
@@ -71,6 +72,7 @@ public function testSendsCorrectHeaders(): void
         $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']);
+        $this->assertEquals('my-instance-id', $headers['X-LaunchDarkly-Instance-Id']);
     }
 
     public function wrapperProvider(): array