diff --git a/.github/actions/build-docs/action.yml b/.github/actions/build-docs/action.yml
index 29497946..3951ae55 100644
--- a/.github/actions/build-docs/action.yml
+++ b/.github/actions/build-docs/action.yml
@@ -17,4 +17,4 @@ runs:
           --ignore-tags psalm-return \
           --visibility public \
           --defaultpackagename "LaunchDarkly" \
-          --title "LaunchDarkly PHP SDK 6.5.2" # x-release-please-version
+          --title "LaunchDarkly PHP SDK 6.6.0" # x-release-please-version
diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 23dcfcf2..2f0a9c43 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "6.5.2"
+  ".": "6.6.0"
 }
diff --git a/CHANGELOG.md b/CHANGELOG.md
index fd99d133..9d9a9026 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,18 @@
 
 All notable changes to the LaunchDarkly PHP SDK will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org).
 
+## [6.6.0](https://github.com/launchdarkly/php-server-sdk/compare/6.5.2...6.6.0) (2025-04-08)
+
+
+### Features
+
+* Include instance ID header across API requests ([#221](https://github.com/launchdarkly/php-server-sdk/issues/221)) ([4f87f4c](https://github.com/launchdarkly/php-server-sdk/commit/4f87f4cd37bdf93b61f8c9270d3d5821decf8b1b))
+
+
+### Miscellaneous Chores
+
+* Add user agent to .sdk_metadata.json ([#224](https://github.com/launchdarkly/php-server-sdk/issues/224)) ([cffa148](https://github.com/launchdarkly/php-server-sdk/commit/cffa148695755d45f2dd2b1fa1dd577133f22d8e))
+
 ## [6.5.2](https://github.com/launchdarkly/php-server-sdk/compare/6.5.1...6.5.2) (2025-03-25)
 
 
diff --git a/src/LaunchDarkly/LDClient.php b/src/LaunchDarkly/LDClient.php
index 72d2a172..56cacdf0 100644
--- a/src/LaunchDarkly/LDClient.php
+++ b/src/LaunchDarkly/LDClient.php
@@ -40,7 +40,7 @@ class LDClient
      * The current SDK version.
      * @var string
      */
-    const VERSION = '6.5.2'; // x-release-please-version
+    const VERSION = '6.6.0'; // x-release-please-version
 
     protected string $_sdkKey;
     protected string $_baseUri;