Deprecation: #78733 - CallUserFunction "&" token for singleton objects

See Issue #78733

Description

The method GeneralUtility::callUserFunction() allows to send the callee (the user-defined function) to be prepended with a "&" before the method name to add the instantiated object to a "singleton" pool during a single request. This functionality has been marked as deprecated as it can easily be solved by implementing a class as singleton.

This way, the object is always a singleton, even when it is called via GeneralUtility::makeInstance().

Impact

Calling callUserFunction() with a "&" symbol will trigger a deprecation log entry.

Affected Installations

Any installation with a hook or user function which is registered with an ampersand "&" symbol.

Migration

The class of the user function / method can implement the SingletonInterface to achieve the same behaviour.