feat(php): Let people know that the PHP SDKs support DS (#5581)

Author: cleptric

src/includes/sampling-supported-sdks.mdx

@@ -4,3 +4,4 @@
 - Android: 6.2.1 or later
 - React Native: 4.3.0 or later
 - Dart and Flutter: 6.11.0 or later
+- PHP: 3.9.0 or later