undefined class constant (function)

Hello; I am trying to call either of two defined functions in a class def that are assigned to associative array items. $_proc = []; $_proc['sync'] = self::listSync; $_proc['async'] = self::listAsync; return $_proc[$_type]($_fileLst); "Fatal error: undefined class constant listSync...." private function listSync(<arg>) { // code, return result; } private function listAsync(<arg>) { // code, return result; } I thought that if I did it like: $_proc['sync'] = self::listSync() self::listSync would be run and $_proc['sync'] would contain what is returned. So, trying to avoid a round of if/else if or switch block, how would I do this? I thought that this might work like javascript (I know javascript is a different language but is mostly translatable to php). In js a function can be assigned to a variable and called via the variable at a later time. While composing this it might not be a bad idea that $_proc[<label>] would contain the result of the function. So I probably answered my own question for the task at hand. But allowing the $_proc items to hold function result means that both function would have to be run so a statement like return $_proc[$_type]; would work. For future reference, how would I assign a function to a variable for delayed execution in this situation? Thanks for time and attention JK.