从另一个函数调用时函数不返回任何内容

I have a function (function a) which calls another function (function b) that then executes a curl request. It works fine and returns all data. But if I try and call function a from function c, no data is returned.

class this_class {

 public function a() {

    $data = $this->b();

    return $data; // returns $json
 }

 public function c() {

    $more_data = $this->a();

    return $more_data; // returns nothing
 }

 public function b() {

    // curl stuff

    return $json; // returns $json
 }