Ask the experts to help modify a recursive function to make it return values

Please help me modify the recursive function so that it has a returned value. Currently, I have a recursive function. its function is to echo the ID value in a multi-dimensional array and display it as "1, 2, 3, 4" on the page, 5... "PHPcodefunctionarr_fun ($ arr) {if (is_array ($ arr) {ask the experts to modify the recursive function and let it return values.
At present, I have a recursive function, which is used to echo the ID value in a multi-dimensional array.
Echo is displayed as "1, 2, 3, 4, 5..." on the page ..."

PHP code

  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.",";        }  } 

Because I need to use this string of IDs from echo
Failed to assign a value to $ s = arr_fun ($ array_menu); note: $ array_menu is a dynamic multi-dimensional array with dynamic dimensions.
So I think the above method does not return values, so I cannot assign values.
I hope my predecessors will give me some advice and help me modify it. how can I make the returned value "1, 2, 3, 4 ..."?


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PHP code

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;  }