PHP variable returning null. (Laravel)

Am calling function self::func1(self::func2()) from another function create(). it returns a generated code. After execution sometimes it returns as null even though the variable has value. Can anyone help me?

The var_dump($this->randomCode) just before returning function has value. but after that dd($Code) is shown as null.

public function create(){

  $Code = self::func1(self::func2());
  dd($Code); 
}


private function func1($length = 2) {

 $this->randomCode = // here i implemented a rand() logic
 $codeAlreadyExist = DB::table(self::getTableName())->where('short_code', '=', $this->randomCode)->exists();
 if($codeAlreadyExist){
    self::func1(self::func2());
  } else {
    var_dump($this->randomCode);
    return $this->randomCode;
  } 
}

OUTPUT

string(2) "21"
null

via Chebli Mohamed