请高手帮忙修改一上递归函数，让其有返回值

function arr_fun($arr){  
      if(is_array($arr)){  
        foreach($arr as $v){  
           if(is_array($v)){  
           arr_fun($v);  
           }else{  
                echo $arr[id].",";
                break;
           } 
        }  
      }else{  
           echo $arr.",";  
      }
  } 

function arr_fun($arr){  
      $r = '';
      if(is_array($arr)){  
        foreach($arr as $v){  
           if(is_array($v)){  
           $r .= arr_fun($v);  
           }else{  
                $r .= $arr[id].",";
                break;
           } 
        }  
      }else{  
           $r .= $arr.",";  
      }
      return $r;
  }