document explicit php callbacks

ai/backend-context.md

@@ -87,6 +87,8 @@ intentionally unclaimed; the built-in health endpoint is `/up`.
 - Put imports at the top of the file. Do not use inline fully qualified class
   names when a normal `use` statement is clearer.
 - Do not use arrow functions. Use regular anonymous functions.
+- Do not use first-class callable syntax. Use regular anonymous functions for
+  callbacks so the invocation is explicit.
 - Do not add default values to function or constructor parameters. Pass every
   argument explicitly, including nullable arguments.
 - Use descriptive names for classes, methods, parameters, and local