array_map()参数

请问一下array_map([$this,'transform'],$house->toArray())里的[$this,'transform']为何不能写成$this->transform().
private function transformCollection($house){

    return array_map([$this,'transform'],$house->toArray());
}
private function transform($house){
        return[
            'id'=>$house['ID'],
            'area'=>$house['Area'],
            'b_year'=>$house['BuildYear'],
            's_type'=>$house['StructureType'],
            'm_type'=>$house['MoveType'],
        ];
}